Two-dimensional transition metal dichalcogenides (TMDs), an emerging family of layered materials, have provided researchers a fertile ground for harvesting fundamental science and emergent applications. TMDs can contain a number of different structural defects in their crystal lattices which significantly alter their physico-chemical properties. Having structural defects can be either detrimental or beneficial, depending on the targeted application. Therefore, a comprehensive understanding of structural defects is required. Here we review different defects in semiconducting TMDs by summarizing: (i) the dimensionalities and atomic structures of defects; (ii) the pathways to generating structural defects during and after synthesis and, (iii) the effects of having defects on the physico-chemical properties and applications of TMDs. Thus far, significant progress has been made, although we are probably still witnessing the tip of the iceberg. A better understanding and control of defects is important in order to move forward the field of Defect Engineering in TMDs. Finally, we also provide our perspective on the challenges and opportunities in this emerging field.

Surface functionalization of metallic and semiconducting 2D transition metal dichalcogenides (TMDs) have mostly relied on physi- and chemi-sorption at defect sites, which can diminish the potential applications of the decorated 2D materials, as structural defects can have substantial drawbacks on the electronic and optoelectronic characteristics. Here, we demonstrate a spontaneous defect-free functionalization method consisting of attaching Au single atoms to monolayers of semiconducting MoS2(1H) via S-Au-Cl coordination complexes. This strategy offers an effective and controllable approach for tuning the Fermi level and excitation spectra of MoS2 via p-type doping and enhancing the thermal boundary conductance of monolayer MoS2, thus promoting heat dissipation. The coordination-based method offers an effective and damage-free route of functionalizing TMDs and can be applied to other metals and used in single-atom catalysis, quantum information devices, optoelectronics, and enhanced sensing.

Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) can be easily synthesized on SiO2/Si substrates by chemical vapor deposition (CVD). However, for practical applications, those 2D crystals usually need to be retrieved and placed onto target substrates. Hence, a robust and effective transfer process is required. Currently, the most widely used approach for transferring CVD-grown TMDs involves the spin-coating of a poly­(methyl methacrylate) (PMMA) support layer, followed by the wet etching of the SiO2 layer in hot NaOH. This transfer process often causes substantial accumulation of polymer residues as well as severe structural damage of TMDs induced during the etching of substrates at elevated temperatures. In this work, we present an alternative approach for the transfer of CVD-grown TMDs that can address the issues mentioned above. In this process, we replaced PMMA with cellulose acetate (CA) as a support layer and used buffered oxide etch (BOE) as an effective room-temperature etchant for SiO2. The CA-transferred TMDs exhibit well-preserved structural integrity and unaltered optical properties as well as largely improved microscale and nanoscale cleanliness with reduced wrinkles and cracks. Furthermore, we integrated our CA-transfer method with a deterministic positioning system that allowed microprecision transfer of the TMD layers. For example, a WS2–MoS2 vertical heterojunction with an electronically coupled and uniform interface was successfully created. The CA-transfer technique developed in this work represents a cleaner alternative to the PMMA-transfer method, thus permitting atomic resolution characterizations and the implementation of novel applications of CVD-grown TMDs and their heterostructures.

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