Interplay Between Cr Dopants and Vacancy Clustering in the Structural and Optical Properties of WSe2

Engineering 2D Material Exciton Line Shape with Graphene/h-BN Encapsulation

Control over the optical properties of atomically thin two-dimensional (2D) layers, including those of transition metal dichalcogenides (TMDs), is needed for future optoelectronic applications. Here, the near-field coupling between TMDs and graphene/graphite is used to engineer the exciton line shape and charge state. Fano-like asymmetric spectral features are produced in WS2, MoSe2, and WSe2 van der Waals heterostructures combined with graphene, graphite, or jointly with hexagonal boron nitride (h-BN) as supporting or encapsulating layers. Furthermore, trion emission is suppressed in h-BN encapsulated WSe2/graphene with a neutral exciton red shift (44 meV) and binding energy reduction (30 meV). The response of these systems to electron beam and light probes is well-described in terms of 2D optical conductivities of the involved materials. Beyond fundamental insights into the interaction of TMD excitons with structured environments, this study opens an unexplored avenue toward shaping the spectral profile of narrow optical modes for application in nanophotonic devices.

Structure modulation of two-dimensional transition metal chalcogenides: recent advances in methodology, mechanism and applications

Together with the development of two-dimensional (2D) materials, transition metal dichalcogenides (TMDs) have become one of the most popular series of model materials for fundamental sciences and practical applications. Due to the ever-growing requirements of customization and multi-function, dozens of modulated structures have been introduced in TMDs. In this review, we present a systematic and comprehensive overview of the structure modulation of TMDs, including point, linear and out-of-plane structures, following and updating the conventional classification for silicon and related bulk semiconductors. In particular, we focus on the structural characteristics of modulated TMD structures and analyse the corresponding root causes. We also summarize the recent progress in modulating methods, mechanisms, properties and applications based on modulated TMD structures. Finally, we demonstrate challenges and prospects in the structure modulation of TMDs and forecast potential directions about what and how breakthroughs can be achieved.

Patterning and doping of transition metals in tungsten dichalcogenides

Substitutional transition metal doping in two-dimensional (2D) layered dichalcogenides is of fundamental importance in manipulating their electrical, excitonic, magnetic, and catalytic properties through the variation of the d-electron population. Yet, most doping strategies are spatially global, with dopants embedded concurrently during the synthesis. Here, we report an area-selective doping scheme for W-based dichalcogenide single layers, in which pre-patterned graphene is used as a reaction mask in the high-temperature substitution of the W sublattice. The chemical inertness of the thin graphene layer can effectively differentiate the spatial doping reaction, allowing for local manipulation of the host 2D materials. Using graphene as a mask is also beneficial in the sense that it also acts as an insertion layer between the contact metal and the doped channel, capable of depinning the Fermi level for low contact resistivity. Tracing doping by means of chalcogen labelling, deliberate Cr embedment is found to become energetically favorable in the presence of chalcogen deficiency, assisting the substitution of the W sublattice in the devised chemical vapor doping scheme. Atomic characterization using scanning transmission electron microscopy (STEM) shows that the dopant concentration is controllable and varies linearly with the reaction time in the current doping approach. Using the same method, other transition metal atoms such as Mo, V, and Fe can also be doped in the patterned area.

Upconversion of Light into Bright Intravalley Excitons via Dark Intervalley Excitons in hBN-Encapsulated WSe2 Monolayers

Semiconducting monolayers of transition-metal dichalcogenides are outstanding platforms to study both electronic and phononic interactions as well as intra- and intervalley excitons and trions. These excitonic complexes are optically either active (bright) or inactive (dark) due to selection rules from spin or momentum conservation. Exploring ways of brightening dark excitons and trions has strongly been pursued in semiconductor physics. Here, we report on a mechanism in which a dark intervalley exciton upconverts light into a bright intravalley exciton in hBN-encapsulated WSe₂ monolayers. Excitation spectra of upconverted photoluminescence reveals resonances at energies 34.5 and 46.0 meV below the neutral exciton in the nominal WSe₂ transparency range. The required energy gains are theoretically explained by cooling of resident electrons or by exciton scattering with Λ- or K-valley phonons. Accordingly, an elevated temperature and a moderate concentration of resident electrons are necessary for observing the upconversion resonances. The interaction process observed between the inter- and intravalley excitons elucidates the importance of dark excitons for the optics of two-dimensional materials.

Single-Atom Engineering to Ignite 2D Transition Metal Dichalcogenide Based Catalysis: Fundamentals, Progress, and Beyond

Single-atom catalysis has been recognized as a pivotal milestone in the development history of heterogeneous catalysis by virtue of its superior catalytic performance, ultrahigh atomic utilization, and well-defined structure. Beyond single-atom protrusions, two more motifs of single-atom substitutions and single-atom vacancies along with synergistic single-atom motif assemblies have been progressively developed to enrich the single-atom family. On the other hand, besides traditional carbon material based substrates, a wide variety of 2D transitional metal dichalcogenides (TMDs) have been emerging as a promising platform for single-atom catalysis owing to their diverse elemental compositions, variable crystal structures, flexible electronic structures, and intrinsic activities toward many catalytic reactions. Such substantial expansion of both single-atom motifs and substrates provides an enriched toolbox to further optimize the geometric and electronic structures for pushing the performance limit. Concomitantly, higher requirements have been put forward for synthetic and characterization techniques with related technical bottlenecks being continuously conquered. Furthermore, this burgeoning single-atom catalyst (SAC) system has triggered serial scientific issues about their changeable single atom-2D substrate interaction, ambiguous synergistic effects of various atomic assemblies, as well as dynamic structure-performance correlations, all of which necessitate further clarification and comprehensive summary. In this context, this Review aims to summarize and critically discuss the single-atom engineering development in the whole field of 2D TMD based catalysis covering their evolution history, synthetic methodologies, characterization techniques, catalytic applications, and dynamic structure-performance correlations. In situ characterization techniques are highlighted regarding their critical roles in real-time detection of SAC reconstruction and reaction pathway evolution, thus shedding light on lifetime dynamic structure-performance correlations which lay a solid theoretical foundation for the whole catalytic field, especially for SACs.

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

Nitric oxide electroreduction reaction (NOER) is an efficient method for NH₃ synthesis and NO_x-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@V_S). 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@V_S and Au-MoS₂@V_S). At high NO coverage, N₂O/N₂ is thermodynamically favored. Meanwhile, the competing hydrogen evolution reaction (HER) is suppressed. Thus, the Pt-MoS₂@V_S 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.

Deep Learning-Assisted Quantification of Atomic Dopants and Defects in 2D Materials

Atomic dopants and defects play a crucial role in creating new functionalities in 2D transition metal dichalcogenides (2D TMDs). Therefore, atomic-scale identification and their quantification warrant precise engineering that widens their application to many fields, ranging from development of optoelectronic devices to magnetic semiconductors. Scanning transmission electron microscopy with a sub-Å probe has provided a facile way to observe local dopants and defects in 2D TMDs. However, manual data analytics of experimental images is a time-consuming task, and often requires subjective decisions to interpret observed signals. Therefore, an approach is required to automate the detection and classification of dopants and defects. In this study, based on a deep learning algorithm, fully convolutional neural network that shows a superior ability of image segmentation, an efficient and automated method for reliable quantification of dopants and defects in TMDs is proposed with single-atom precision. The approach demonstrates that atomic dopants and defects are precisely mapped with a detection limit of ≈1 × 1012 cm-2 , and with a measurement accuracy of ≈98% for most atomic sites. Furthermore, this methodology is applicable to large volume of image data to extract atomic site-specific information, thus providing insights into the formation mechanisms of various defects under stimuli.

Investigations of Electron-Electron and Interlayer Electron-Phonon Coupling in van der Waals hBN/WSe2/hBN Heterostructures by Photoluminescence Excitation Experiments

Monolayers of transition metal dichalcogenides (TMDs) with their unique physical properties are very promising for future applications in novel electronic devices. In TMDs monolayers, strong and opposite spin splittings of the energy gaps at the K points allow for exciting carriers with various combinations of valley and spin indices using circularly polarized light, which can further be used in spintronics and valleytronics. The physical properties of van der Waals heterostructures composed of TMDs monolayers and hexagonal boron nitride (hBN) layers significantly depend on different kinds of interactions. Here, we report on observing both a strong increase in the emission intensity as well as a preservation of the helicity of the excitation light in the emission from hBN/WSe₂/hBN heterostructures related to interlayer electron-phonon coupling. In combined low-temperature (T = 7 K) reflectivity contrast and photoluminescence excitation experiments, we find that the increase in the emission intensity is attributed to a double resonance, where the laser excitation and the combined Raman mode A'₁ (WSe₂) + ZO (hBN) are in resonance with the excited (2s) and ground (1s) states of the A exciton in a WSe₂ monolayer. In reference to the 2s state, our interpretation is in contrast with previous reports, in which this state has been attributed to the hybrid exciton state existing only in the hBN-encapsulated WSe₂ monolayer. Moreover, we observe that the electron-phonon coupling also enhances the helicity preservation of the exciting light in the emission of all observed excitonic complexes. The highest helicity preservation of more than 60% is obtained in the emission of the neutral biexciton and negatively charged exciton (trion) in its triplet state. Additionally, to the best of our knowledge, the strongly intensified emission of the neutral biexciton XX⁰ at double resonance condition is observed for the first time.

Ferromagnetic Order at Room Temperature in Monolayer WSe2 Semiconductor via Vanadium Dopant

Diluted magnetic semiconductors including Mn-doped GaAs are attractive for gate-controlled spintronics but Curie transition at room temperature with long-range ferromagnetic order is still debatable to date. Here, the room-temperature ferromagnetic domains with long-range order in semiconducting V-doped WSe2 monolayer synthesized by chemical vapor deposition are reported. Ferromagnetic order is manifested using magnetic force microscopy up to 360 K, while retaining high on/off current ratio of ≈105 at 0.1% V-doping concentration. The V-substitution to W sites keeps a V-V separation distance of 5 nm without V-V aggregation, scrutinized by high-resolution scanning transmission electron microscopy. More importantly, the ferromagnetic order is clearly modulated by applying a back-gate bias. The findings open new opportunities for using 2D transition metal dichalcogenides for future spintronics.

Spatially Controlled Fabrication and Mechanisms of Atomically Thin Nanowell Patterns in Bilayer WS2 Using in Situ High Temperature Electron Microscopy

We show controlled production of atomically thin nanowells in bilayer WS₂ using an in situ heating holder combined with a focused electron beam in a scanning transmission electron microscope (STEM). We systematically study the formation and evolvement mechanism involved in removing a single layer of WS₂ within a bilayer region with 2 nm accuracy in location and without punching through to the other layer to create a hole. Best results are found when using a high temperature of 800 °C, because it enables thermally activated atomic migration and eliminates the interference from surface carbon contamination. We demonstrate precise control over spatial distributions with 5 nm accuracy of patterning and the width of nanowells adjustable by dose-dependent parameters. The mechanism of removing a monolayer of WS₂ within a bilayer region is different than removing equivalent sections in a monolayer film due to the van der Waals interaction of the underlying remaining layer in the bilayer system that stabilizes the excess W atom stoichiometry within the edges of the nanowell structure and facilitates expansion. This study offers insights for the nanoengineering of nanowells in two-dimensional (2D) transitional metal dichalcogenides (TMDs), which could hold potential as selective traps to localize 2D reactions in molecules and ions, underpinning the broader utilization of 2D material membranes.

Atomic-Scale Structural Modification of 2D Materials

2D materials have attracted much attention since the discovery of graphene in 2004. Due to their unique electrical, optical, and magnetic properties, they have potential for various applications such as electronics and optoelectronics. Owing to thermal motion and lattice growth kinetics, different atomic-scale structures (ASSs) can originate from natural or intentional regulation of 2D material atomic configurations. The transformations of ASSs can result in the variation of the charge density, electronic density of state and lattice symmetry so that the property tuning of 2D materials can be achieved and the functional devices can be constructed. Here, several kinds of ASSs of 2D materials are introduced, including grain boundaries, atomic defects, edge structures, and stacking arrangements. The design strategies of these structures are also summarized, especially for atomic defects and edge structures. Moreover, toward multifunctional integration of applications, the modulation of electrical, optical, and magnetic properties based on atomic-scale structural modification are presented. Finally, challenges and outlooks are featured in the aspects of controllable structure design and accurate property tuning for 2D materials with ASSs. This work may promote research on the atomic-scale structural modification of 2D materials toward functional applications.

Chemical vapor deposition growth of sub-centimeter single crystal WSe2 monolayer by NaCl-assistant

Monolayer WSe₂ exhibits unique optical and electronic properties, showing great potential applications in functional integrated devices, such as electronic devices and optoelectronics. Understanding the growth behavior and process are the key points for the salt-assisted growth of large domain WSe₂ monolayers, it is also very important for its further application in on-chip laser and opto-devices. Here, we report a NaCl-assistant method for controlled growth of single crystal monolayer WSe₂ with a domain size up to 0.57 mm on SiO₂/Si substrate. Atomic-resolution scanning transmission electron microscopy reveals that the Se₁ and Se₂ vacancy point defects are the main defect type of those materials. The growth behavior of the salt-assisted method have been systemly investigated. The loading mass of NaCl powder prefers to be less with the controllable vapor process. The flow of hydrogen gas was also preferred to be suitable with a weak etching effect. The morphology of monolayer WSe₂ shows a sensitive temperature dependence evolution with the growth temperature increasing. A screw dislocation growth behavior with 15° angle is also observed with the NaCl-assistant method. The results provide a deep understanding of the mechanism for the NaCl-assistant growth of large size monolayer WSe₂.

Differentiating Polymorphs in Molybdenum Disulfide via Electron Microscopy

The presence of rich polymorphs and stacking polytypes in molybdenum disulfide (MoS₂ ) endows it with a diverse range of electrical, catalytic, optical, and magnetic properties. This has stimulated a lot of interest in the unique properties associated with each polymorph. Most techniques used for polymorph identification in MoS₂ are macroscopic techniques that sample averaged properties due to their limited spatial resolution. A reliable way of differentiating the atomic structure of different polymorphs is needed in order to understand their growth dynamics and establish the correlation between structure and properties. Herein, the use of electron microscopy for identifying the atomic structures of several important polymorphs in MoS₂ , some of which are the subjects of mistaken assignment in the literature, is discussed. In particular, scanning transmission electron microscopy-annular dark field imaging has emerged as the most effective and reliable approach for identifying the different phases in MoS₂ and other 2D materials because its images can be directly correlated to the atomic structures. Examples of the identification of polymorphs grown under different conditions in molecular beam epitaxy or chemical vapor deposition, for example, 3R, 1T, 1T'-phases, and 1T'-edges, are presented, including their atomic structures, fascinating properties, growth methods, and corresponding thermodynamic stabilities.

Dynamic tungsten diselenide nanomaterials: supramolecular assembly-induced structural transition over exfoliated two-dimensional nanosheets

Supramolecular polymers can easily control the lamellar microstructures on exfoliated tungsten diselenide nanosheets.

A simple and effective method for direct exfoliation of tungsten diselenide (WSe₂) into few-layered nanosheets has been successfully developed by employing a low molecular weight adenine-functionalized supramolecular polymer (A-PPG). In this study, we discover A-PPG can self-assemble into a long-range, ordered lamellar microstructure on the surface of WSe₂ due to the efficient non-covalent interactions between A-PPG and WSe₂. Morphological and light scattering studies confirmed the dynamic self-assembly behavior of A-PPG has the capacity to efficiently manipulate the transition between contractile and extended lamellar microstructures on the surface of metallic 1T-phase and semiconducting 2H-phase WSe₂ nanosheets, respectively. The extent of WSe₂ exfoliation can be easily controlled by systematically adjusting the amount of A-PPG in the composites, to obtain nanocomposites with the desired functional characteristics. In addition, the resulting composites possess unique liquid–solid phase transition behavior and excellent thermoreversible properties, revealing the self-assembled lamellar structure of A-PPG functions as a critical factor to manipulate and tailor the physical properties of exfoliated WSe₂. This newly developed method of producing exfoliated WSe₂ provides a useful conceptual and potential framework for developing WSe₂-based multifunctional nanocomposites to extend their application in solution-processed semiconductor devices.

Interlayer coupling and the phase transition mechanism of stacked MoS2/TaS2 heterostructures discovered using temperature dependent Raman and photoluminescence spectroscopy

Ultrathin 1T (tetragonal)-TaS2 and monolayer MoS2 heterostructures were prepared to study their phase transition (PT) mechanisms and band structure modulation. The temperature dependency of photoluminescence (PL) and Raman spectra was utilized to study interlayer coupling and band structure. The PL results indicate that the band structure of MoS2/TaS2 heterostructures undergoes a sharp change at 214 K. This is attributed to the PT of 1T-TaS2 from a Mott insulator state to a metastable state. In addition, the temperature dependency of the MoS2/TaS2 Raman spectra illustrates that the phonon vibration of the heterojunction is softened due to the effect of interlayer coupling. The present work could provide an avenue to create material systems with abundant functionalities and physical effects.

Synthesizing 1T-1H Two-Phase Mo1-xWxS2 Monolayers by Chemical Vapor Deposition

1T-1H metal-semiconductor interfaces in two-dimensional (2D) transition-metal dichalcogenides (TMDs) play a crucial role in utilizing the band gaps of TMDs for applications in electronic devices. Although the 1T-1H two-phase structure has been observed in exfoliated 2D nanosheets and chemically or physically treated TMDs, it cannot in principle be achieved in large-scale TMD monolayers grown by chemical vapor deposition (CVD), which is a fabrication method for electronic device applications, because of the metastable nature of the 1T phase. In this study we report CVD growth of 1T-1H two phase TMD monolayers by controlling thermal strains and alloy compositions. It was found that in-plane thermal strains arising from the difference in thermal expansion coefficients between TMD monolayers and substrates can drive the 1H to 1T transition during cooling after CVD growth. Moreover, grain boundaries in the 2D crystals act as the nucleation sites of the 1T phase and the lattice strain perturbations from alloying noticeably promote the formation of the metastable 1T phase. This work has an important implication in tailoring structure and properties of CVD grown 2D TMDs by phase engineering.