Formation of crystalline iron-mono-selenide in the restricted one-dimensional space of single-walled carbon nanotubes promoted with H2 annealing

As layered materials beyond graphene discovered after 2004, two-dimensional (2D) transition metal dichalcogenides (TMDs) are vital to fundamental research and practical applications, owing to their unique crystal structures and excellent properties, as well as diversity of the electronic band structures. 2D TMDs have played an important role in electronics, optoelectronics, energy storage and catalysis. To meet with the increasing requirements of programmable and function-integrated devices, property modulation could be concerned as one of the most essential strategies for 2D TMDs. In comparison with conventional external electrical field induction, strain regulation could be much more efficient. In detail, external induction through the electrical field leads to the electron delocalization along the field direction and then induces the transformation of band structures, but electrical field exhibits small modulation for monolayer TMDs. On the contrary, the strain regulation shows excellent tuning efficiency for continuous and reversible modulation. As a result, the strain regulation has become a commonly-used strategy for property tuning of 2D TMDs. On the basis of the lattice transformation, the strain regulation of 2D TMDs can lead to different overlapping of d and p orbits of metal and chalcogen atoms, and then affect the electronic structures of 2D TMDs. Therefore, it can be applied to electronics, optoelectronics, magnetic devices and piezoelectronics. The strategies of introducing strains to 2D TMDs are classified into lattice induction, local deformation, macroscopic regulation and so on. Lattice induction is attributable to the structural distortions and mismatches, including atomic defect induction and lattice mismatch induction. The former one demonstrates that the microenvironment affected by atomic vacancies and doping atoms can introduce strain to 2D TMDs. The latter one means that the lattice mismatches between two materials (between two different TMDs in a heterostructure or between a TMD and the substrate) can result in lattice distortion and then induce the strain. However, strain introduced through lattice induction is fixed and difficult to achieve the reversible modulation. Local deformation refers to the morphological transformation at the scale of several micrometers, which can be induced by the bubbles and wrinkles of 2D TMDs, external forces of tips, as well as patterned substrates. Generally, large but nonuniform strain can be introduced to 2D TMDs through the local deformation, which results in the funnel effects and then induces large property variation. Furthermore, macroscopic regulation introduces the strains to the lattices, including the deformation of flexible substrates (bend, tension and compression), external pressure (applied by the diamond anvil cell) and thermal expansion coefficient mismatch. Macroscopic regulation would be compatible with industrial manufacture to achieve strain regulation on an extremely large scale in the future. There are some other ways to achieve strain regulation such as the design of special stack structures and the induction of the external electrical field. After summarizing the methods of introducing strain to 2D TMDs, we presented the applications based on strain regulation, such as field effect transistors, flexible photodetectors and strain sensors. Finally, we pointed out the further development and challenges of strain regulation of 2D TMDs.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been emerged as a new class of van der Waals materials since the successful isolation of graphene. The strong spin-orbit coupling (SOC) and two-dimensionality give rise to plenty of novel physics including metal-insulator transition, charge density wave (CDW), valleytronics, quantum spin Hall effect, and unconventional superconductivity, which make TMDCs an ideal platform to study the fundamental physics and potential applications. In this review, we firstly introduce the crystal structure of 2D TMDCs materials. Then, we summarize the recent progress in the synthesis, novel physical properties, and applications of 2D TMDCs materials. Finally, a summary and an outlook on the topological superconductivity in this field are presented. 2D TMDCs have a chemical formula of MX2 (M=W, Mo and X=Te, Se, S) with a layered crystal structure. Depending on the coordination environments of M, TMDCs can crystallize in a variety of polytypic structures such as 2 H , 1 T , 1 T ′, and T d phases. In the monolayer 2 H -TMDCs, the breaking of an in-plane mirror symmetry and the presence of the out-of-plane mirror symmetry lead to an Ising spin–orbit coupling (SOC), which serves as an effective out-of-plane field acting on the copper pair and pins the electron spins to out-of-plane directions. This is called Ising superconductivity, which has been observed in gated MoS2, monolayer NbSe2 and TaS2. However, due to the inversion symmetry in monolayer 1 T ′-TMDCs, the introduction of SOC makes them a class of large-gap quantum spin Hall insulators such as monolayer 1 T ′-WTe2. Therefore, if we could consecutively tailor the TMDCs’ structure from 2 H to T d phase, the long-sought topological superconductivity may be realized in one substance by incorporating superconductivity and quantum spin Hall effect together. To explore the extraordinary physics and nanodevice applications, we develop a universal molten-salt-assisted chemical vapor deposition (CVD) method to prepare atomically thin TMDCs, including high-quality 2D superconductors such as monolayer MoTe2 and NbSe2. With the powerful sample growth technique , we demonstrate for the first time that a consecutive structural phase transition from T d to 1 T ′ to 2 H polytype can be realized by increasing the Se concentration in Se-substituted MoTe2. More importantly, the Se-substitution is found to dramatically enhance the superconductivity of the MoTe2 thin film, which is interpreted as the introduction of two-band superconductivity. Furthermore, in bilayer 1 T d-MoTe2, we find that the in-plane upper critical field goes beyond the Pauli paramagnetic limit and shows an emergent two-fold symmetry, which is different from the isotropic in-plane upper critical field in 2 H -TMDCs. We show that this is a result of a new type of asymmetric SOC in 1 T d-TMDCs, which has expanded the well-known Ising SOC. The polytypic structures and strong SOC in 2D TMDCs have led to a variety of novel physics and applications. Recent theoretical works have already shown that 2D TMDCs can be a platform to search for topological superconductivity. With the further development of sample preparation, 2D TMDCs will play an important role in realization of topological quantum computation.

Morphology, Photoluminescence and Photovoltaic Properties of Laser Processed ZnO/carbon Nanotube Nanohybrids

Achieving low-defect interfaces between industry compatible oxide dielectrics and two-dimensional (2D) transition metal dichalcogenide (TMD) semiconducting channels remains challenging but is the key for unlocking their potential for high-performance field-effect transistors (FETs). This perspective analyses the state of the art on 2D TMD and dielectric interfaces, highlighting key challenges in depositing oxide dielectrics on top of atomically thin TMD semiconductors. We provide a critical summary of exotic dielectric materials reported for FETs based on 2D TMD—including methods for their integration with TMDs. We also discuss characterization methods used to provide insight into properties of the interface between 2D TMDs and the dielectrics—in particular for identification of defects and how they influence the device performance. In addition, we provide a set of guidelines for evaluating and selecting dielectrics for 2D TMDs with the aim of realizing high-performance FETs using industry compatible approaches.

Exposure to oxygen alters the physical and chemical properties of two-dimensional (2D) transition metal dichalcogenides (TMDs). In particular, oxygen in the ambient may influence the device stability of 2D TMDs over time. Engineering the doping of 2D TMDs, especially hole doping is highly desirable towards their device function. Herein, controllable oxygen-induced p-type doping in a range of hexagonal (MoTe2, WSe2, MoSe2 and PtSe2) and pentagonal (PdSe2) 2D TMDs are demonstrated. Scanning tunneling microscopy, electrical transport and X-ray photoelectron spectroscopy are used to probe the origin of oxygen-derived hole doping. Three mechanisms are postulated that contribute to the hole doping in 2D TMDs, namely charge transfer from absorbed oxygen molecules, surface oxides, and chalcogen atom substitution. This work provides insights into the doping effects of oxygen, enabling the engineering of 2D TMDs properties for nanoelectronic applications.

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.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have gained much attention in virtue of their various atomic configurations and band structures. Apart from those thermodynamically stable phases, plenty of metastable phases exhibit interesting properties. To obtain 2D TMDs with specific phases, it is important to develop phase engineering strategies including phase transition and phase-selective synthesis. Phase transition is a conventional method to transform one phase to another, while phase-selective synthesis means the direct fabrication of the target phases for 2D TMDs. In this review, we introduce the structures and stability of 2D TMDs with different phases. Then, we summarize the detailed processes and mechanism of the traditional phase transition strategies. Moreover, in view of the increasing demand of high-phase purity TMDs, we present the advanced phase-selective synthesis strategies. Finally, we underline the challenges and outlooks of phase engineering of 2D TMDs in two aspects---high phase purity and excellent controllability. This review may promote the development of controllable phase engineering for 2D TMDs and even other 2D materials toward both fundamental studies and practical applications.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) with fascinating electronic energy band structures, rich valley physical properties and strong spin–orbit coupling have attracted tremendous interest, and show great potential in electronic, optoelectronic, spintronic and valleytronic fields. Stacking 2D TMDs have provided unprecedented opportunities for constructing artificial functional structures. Due to the low cost, high yield and industrial compatibility, chemical vapor deposition (CVD) is regarded as one of the most promising growth strategies to obtain high-quality and large-area 2D TMDs and heterostructures. Here, state-of-the-art strategies for preparing TMDs details of growth control and related heterostructures construction via CVD method are reviewed and discussed, including wafer-scale synthesis, phase transition, doping, alloy and stacking engineering. Meanwhile, recent progress on the application of multi-functional devices is highlighted based on 2D TMDs. Finally, challenges and prospects are proposed for the practical device applications of 2D TMDs.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have garnered widespread interest in the scientific community and industry for their exceptional physical and chemistry properties, and great potential for applications in diverse fields including (opto)electronics, electrocatalysis, and energy storage. Chemical vapor deposition (CVD) is one of the most compelling growth methods for the scalable growth of high-quality 2D TMDs. However, the conventional CVD process for synthesis of 2D TMDs still encounters significant challenges, primarily attributed to the high melting point of precursor powders, and achieving a uniform distribution of precursor atmosphere on the substrate to obtain controllable smaple domains is difficult. The spin-coating precursor mediated chemical vapor deposition (SCVD) strategy provides refinement over traditional methods by eliminating the use of solid precursors and ensuring a more clean and uniform distribution of the growth material on the substrate. Additionally, the SCVD process allows fine-tuning of material thickness and purity by manipulating solution composition, concentration, and the spin coating process. This Review presents a comprehensive summary of recent advances in controllable growth of 2D TMDs with a SCVD strategy. First, a series of various liquid precursors, additives, source supply methods, and substrate engineering strategies for preparing atomically thin TMDs by SCVD are introduced. Then, 2D TMDs heterostructures and novel doped TMDs fabricated through the SCVD method are discussed. Finally, the current challenges and perspectives to synthesize 2D TMDs using SCVD are discussed.

Two-dimensional (2D) transition metal dichalcogenide (TMD) nanosheets exhibit remarkable electronic and optical properties. The 2D features, sizable bandgaps and recent advances in the synthesis, characterization and device fabrication of the representative MoS2, WS2, WSe2 and MoSe2 TMDs make TMDs very attractive in nanoelectronics and optoelectronics. Similar to graphite and graphene, the atoms within each layer in 2D TMDs are joined together by covalent bonds, while van der Waals interactions keep the layers together. This makes the physical and chemical properties of 2D TMDs layer-dependent. In this review, we discuss the basic lattice vibrations of 2D TMDs from monolayer, multilayer to bulk material, including high-frequency optical phonons, interlayer shear and layer breathing phonons, the Raman selection rule, layer-number evolution of phonons, multiple phonon replica and phonons at the edge of the Brillouin zone. The extensive capabilities of Raman spectroscopy in investigating the properties of TMDs are discussed, such as interlayer coupling, spin-orbit splitting and external perturbations. The interlayer vibrational modes are used in rapid and substrate-free characterization of the layer number of multilayer TMDs and in probing interface coupling in TMD heterostructures. The success of Raman spectroscopy in investigating TMD nanosheets paves the way for experiments on other 2D crystals and related van der Waals heterostructures.

2D transition metal chalcogenides have attracted tremendous attention due to their novel properties and potential applications. Although 2D transition metal dichalcogenides are easily fabricated due to their layer-stacked bulk phase, 2D transition metal monochalcogenides are difficult to obtain. Recently, a single atomic layer transition metal monochalcogenide (CuSe) with an intrinsic pattern of nanoscale triangular holes is fabricated on Cu(111). The first-principles calculations show that free-standing monolayer CuSe with holes is not stable, while hole-free CuSe is endowed with the Dirac nodal line fermion (DNLF), protected by mirror reflection symmetry. This very rare DNLF state is evidenced by topologically nontrivial edge states situated inside the spin-orbit coupling gaps. Motivated by the promising properties of hole-free honeycomb CuSe, monolayer CuSe is fabricated on Cu(111) surfaces by molecular beam epitaxy and confirmed success with high resolution scanning tunneling microscopy. The good agreement of angle resolved photoemission spectra with the calculated band structures of CuSe/Cu(111) demonstrates that the sample is monolayer CuSe with a honeycomb lattice. These results suggest that the honeycomb monolayer transition metal monochalcogenide can be a new platform to study 2D DNLFs.

Nonlinear Nanophotonics With 2D Transition Metal Dichalcogenides

SummaryAtomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted significant attention owing to their prosperity in material research. The inimitable features of TMDCs triggered the emerging applications in diverse areas. In this review, we focus on the tailored and engineering of the crystal lattice of TMDCs that finally enhance the efficiency of the material properties. We highlight several preparation techniques and recent advancements in compositional engineering of TMDCs structure. We summarize different approaches for TMDCs such as doping and alloying with different materials, alloying with other 2D metals, and scrutinize the technological potential of these methods. Beyond that, we also highlight the recent significant advancement in preparing 2D quasicrystals and alloying the 2D TMDCs with MAX phases. Finally, we highlight the future perspectives for crystal engineering in TMDC materials for structure stability, machine learning concept marge with materials, and their emerging applications.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted great attention as alternatives to graphene with semiconducting band gaps. Mono- or few-layer TMDCs can be prepared by various methods, but regardless of the fabrication methods [such as mechanical exfoliation and chemical vapor deposition (CVD)], TMDCs contain many structural defects, which significantly affect their physical properties and limit their performance in applications. Metallophthalocyanines (MPcs) are organic semiconductors, and as dopants, they are capable of modulating the optical and electrical properties of other semiconducting materials. Here, we report that besides the ability to modulate the optoelectronic properties of 2D TMDCs, MPc molecules can be used to heal defects and improve the physicochemical properties of TMDCs. Doping of planar MPc molecules to TMDCs is achieved by a simple solution dip-coating method and results in a significant improvement in the optical properties and thermal responses of CVD-grown TMDCs, even comparable to those of mechanically exfoliated counterparts. Study of carrier dynamics shows that the adsorption of MPc on the TMDC surface leads to the complete suppression of the mid-gap defect-induced absorption in TMDCs. Furthermore, MPc molecules with a large lateral size are found to effectively reduce the point defects in mechanically exfoliated TMDCs introduced during the preparation process. Our results not only clarify the optoelectronic modulation mechanism of chemical doping but also offer a simple method to control the nanosized defects in 2D TMDCs.

As a hole-transport layer (HTL) material, poly(3,4-ethylenedioxythiophene):polystyrene-sulfonate (PEDOT:PSS) was often criticized for its intrinsic acidity and hygroscopic effect that would in the long run affect the stability of perovskite solar cells (Pero-SCs). As alternatives, herein water-soluble two-dimensional (2D) transition metal dichalcogenides (TMDs), such as MoS2 and WS2 were used as HTLs in p-i-n Pero-SCs. It was found that the content of 1T phase in 2D TMDs HTLs is centrally important to the power conversion efficiencies (PCEs) of Pero-SCs, and the 1T-rich TMDs (as achieved from exfoliation and without postheating) lead to much higher PCEs. More importantly, as PEDOT:PSS was replaced by 2D TMDs, both the PCEs and stability of Pero-SCs were significantly improved. The highest PCEs of 14.35 and 15.00% were obtained for the Pero-SCs with MoS2 and WS2, respectively, whereas the Pero-SCs with PEDOT:PSS showed a highest PCE of only 12.44%. These are up to date the highest PCEs using 2D TMDs as HTLs. After being stored in a glovebox for 56 days, PCEs of the Pero-SCs using MoS2 and WS2 HTLs remained 78 and 72%, respectively, whereas the PCEs of Pero-SCs with PEDOT:PSS almost dropped to 0 over 35 days. This study demonstrates that water-soluble 2D TMDs have great potential for application as new generation of HTLs aiming at high performance and long-term stable Pero-SCs.