Layered Dichalcogenides Research Articles

Engineering of layered metal dichalcogenides: introducing imperfections to make it perfect

AbstractIn the past decade, the surge in research of layered metal dichalcogenides (LMDs) has already demonstrated the tremendous potentiality of this particular category of materials towards technology. But in parallel, it is also established that to make them technology-perfect meticulous engineering to impose ‘imperfections’ within the materials is inevitable. So exploring different LMD with inexorable and appropriate engineering techniques for the enhancement of their functionality is the burning issue for materials scientists. This review comprehensively focuses on different pathways of introducing ‘imperfections’ within various LMDs, mainly by engineering the thickness, morphology, defect, doping and phase. Based on recent progress thickness and shape engineering of LMDs have been discussed with their success and modulation by defect has been examined in detail. Doping and phase engineering of LMDs have also been illustrated with the light of development till now. Finally, challenges and opportunities associated with this research direction are highlighted.

TiO2 Nanorod Array Conformally Coated with a Monolayer MoS2 Film: An Efficient Electrocatalyst for Hydrogen Evolution Reaction

Heterostructures, particularly the shell–core (2D@1D) nanostructure with atomically thin layers of transition metal dichalcogenides, exhibit an excellent electrocatalytic hydrogen evolution reactio...

Charge Instability in Single-Layer TiTe_{2} Mediated by van der Waals Bonding to Substrates.

Single layers of transition metal dichalcogenides are of interest for emergent properties; an often-neglected issue is substrate effects. Our experiments show that the charge density wave in a single-layer TiTe_{2} grown on PtTe_{2} films is strongly suppressed by increasing the PtTe_{2} substrate thickness. Given that the interfacial bonding remains of the weak incommensurate van der Waals type, the observed changes are correlated with a thickness-dependent metallicity transformation in the PtTe_{2} substrate. The results illustrate the crucial role of the substrate in single-layer physics.

Alternative route to a 2D superconductor

Superconductivity Single layers of transition metal dichalcogenides exhibit exotic properties, including superconductivity. The usual route to obtaining such samples is to exfoliate a three-dimensional (3D) crystal. Devarakonda et al. instead grew a superlattice comprising alternating layers of the transition metal dichalcogenide hexagonal NbS2 and the material Ba3NbS5 (see the Perspective by Schoop). The inert Ba3NbS5 layers serve to dissociate the superconducting NbS2 layers from one another, resulting in 2D superconductivity with high carrier mobility. The combination of high mobility and reduced dimensionality may give rise to exotic quantum phases. Science , this issue p. [231][1] see also p. [170][2] [1]: /lookup/doi/10.1126/science.aaz6643 [2]: /lookup/doi/10.1126/science.abd4225

Momentum-resolved observation of ultrafast interlayer charge transfer between the topmost layers of MoS2

How fast is the charge transfer between two layers of transition metal dichalcogenides and where does it take place in momentum space? Two-photon photoemission using high-harmonic probe pulses can answer this question, as the authors demonstrate here for the topmost layers of MoS${}_{2}$. By tuning pump pulses below the topmost-layer gap, they excite electrons in deeper layers and probe only the topmost layer. GW-based tight-binding calculations support the experimental findings and explain why the electron transfer takes place at $\overline{\mathrm{\ensuremath{\Sigma}}}$.

Synthesis, Transfer, and Properties of Layered FeTe2 Nanocrystals.

Different from layered two-dimensional (2D) transition metal dichalcogenides (TMDs), iron dichalcogenides crystallize in the most common three-dimensional pyrite or marcasite structures. Layered iron dichalcogenides are rarely reported and little is known about their structures and properties. Here, layered hexagonal phase iron ditelluride FeTe2 (h-FeTe2) nanocrystals are grown on mica by atmospheric pressure chemical vapor deposition (APCVD) method and are fully characterized by various methods. Like other 2D layered TMD materials, the FeTe2 nanoflakes exhibit regular hexagon, half hexagon, or triangle shapes with a controllable thickness of 6-95 nm and lateral length from a few to tens of micrometers. A simple and effective method is used to transfer the FeTe2 nanoflakes from the mica substrate onto any other substrates without quality deterioration by using polystyrene (PS) as a support polymer, which can also be operated in ethanol or ethylene glycol in a glovebox to avoid contact with water and air. Temperature-dependent electrical transport demonstrates that the FeTe2 nanoflake is a semiconductor with a variable range hopping (VRH) conduction, and its nonsaturated linear magnetoresistance (MR) reaches up to 10.4% under magnetic field of 9 T at 2 K, both probably due to its structure disorders. No signature of magnetic ordering is observed down to 2 K. The CVD growth of this layered FeTe2 represents an addition to the extensive library of 2D materials, particularly iron chalcogenides or alloys. Synthesis, properties, and even doping of phase pure h-FeTe2 call for further study in the future.

MoS2 /Polymer Heterostructures Enabling Stable Resistive Switching and Multistate Randomness.

Resistive random-access memories (ReRAMs) based on transition metal dichalcogenide layers are promising physical sources for random number generation (RNG). However, most ReRAM devices undergo performance degradation from cycle to cycle, which makes preserving a normal probability distribution during operation a challenging task. Here, ReRAM devices with excellent stability are reported by using a MoS2 /polymer heterostructure as active layer. The stability enhancement manifests in outstanding cumulative probabilities for both high- and low-resistivity states of the memory cells. Moreover, the intrinsic values of the high-resistivity state are found to be an excellent source of randomness as suggested by a Chi-square test. It is demonstrated that one of these cells alone can generate ten distinct random states, in contrast to the four conventional binary cells that would be required for an equivalent number of states. This work unravels a scalable interface engineering process for the production of high-performance ReRAM devices, and sheds light on their promising application as reliable RNGs for enhanced cybersecurity in the big data era.

Non-Markovian Spontaneous Emission Dynamics of a Quantum Emitter Near a Transition-Metal Dichalcogenide Layer

We investigate the spontaneous emission dynamics of a two-level quantum emitter near a transition-metal dichalcogenide layer, specifically, a tungsten disulfide (WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) layer. The optical response of the WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer is obtained by electromagnetic calculations using parameters by experimental data. We find that depending on the distance between the quantum emitter, and the WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer the spontaneous decay can be enhanced up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> compared with its free-space value for an emitter with transition dipole moment oriented perpendicular or tangential to the layer. We also perform quantum dynamical calculations, which indicate that the emitter's spontaneous emission dynamics changes from Markovian to weakly non-Markovian to clearly non-Markovian, featuring decaying population oscillations, as the coupling grows from relatively weak to strong, depending on the distance of the emitter to the layer, and the emitter's free-space decay rate. For quantum emitters with fast free-space decay times placed at close distances to the WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer, the ultrastrong coupling regime can be reached, and we find that a significant amount of the initial population of the quantum emitter can remain trapped in it. We also quantify non-Markovianity using different measures. Similar results can also be obtained for other transition-metal dichalcogenide layers, like a tungsten diselenide (WSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) layer.

Use of the Indirect Photoluminescence Peak as an Optical Probe of Interface Defectivity in MoS2

AbstractDefect characterization of 2D materials is a critical aspect for their successful integration in future electronic devices. Here, a simple characterization technique is proposed that opens a path for fast, non‐invasive, quality assessment of transition metal dichalcogenide layers, such as MoS2, and their interfaces. It relates to the correlation between substrate‐induced traps and the indirect‐to‐direct photoluminescence peak ratio. It is shown that the indirect peak is quenched when interfacial trap sites are present. A physical mechanism is proposed to explain the observations based on different recombination mechanisms.

Room-Temperature Valley Polarization in Atomically Thin Semiconductors via Chalcogenide Alloying.

Room-temperature manipulation and processing of information encoded in the electronic valley pseudospin and spin degrees of freedoms lie at the heart of the next technological quantum revolution. In atomically thin layers of transition-metal dichalcogenides (TMDs) with hexagonal lattices, valley-polarized excitations and valley quantum coherence can be generated by simply shining with adequately polarized light. In turn, the polarization states of light can induce topological Hall currents in the absence of an external magnetic field, which underlies the fundamental principle of opto-valleytronics devices. However, demonstration of optical generation of valley polarization at room temperature has remained challenging and not well understood. Here, we demonstrate control of strong valley polarization (valley quantum coherence) at room temperature of up to ∼50% (∼20%) by strategically designing Coulomb forces and spin-orbit interactions in atomically thin TMDs via chalcogenide alloying. We show that tailor making the carrier density and the relative order between optically active (bright) and forbidden (dark) states by key variations on the chalcogenide atom ratio allows full control of valley pseudospin dynamics. Our findings set a comprehensive approach for intrinsic and efficient manipulation of valley pseudospin and spin degree of freedom toward realistic opto-valleytronics devices.

Growth of NiSe2, NiTe2 and alloy NiSe2−xTex nanosheets with tunable shape evolution and chemical composition

The nano-devices based on atomic layers of nickel dichalcogenides, including NiS2, NiSe2 and NiTe2, are rarely studied due to synthetic problems. Recently, ultrathin NiTe2 nanosheets are reported to be grown on SiO2/Si substrate through chemical vapour deposition (CVD). However, the CVD synthesis of other nickel dichalcogenides is still unknown. Here we report the synthesis of NiSe2, NiTe2 and alloy NiSe2−xTex (0 < x < 2) nanosheets through a similar CVD method. We found that the growth temperature has great influences on the shape, size and thickness of as-grown nanosheets. When the growth temperature is about 650 °C, the as-grown NiSe2 and NiTe2 nanosheets are as thin as 2.7 nm and 3.1 nm, respectively, which is equal to three layers of crystal unit. Combining the transmission electron microscopy and high-angle annular dark field scanning transmission electron microscope characterizations, the atomic structures of NiSe2, NiTe2 and alloy NiSe2−xTex are proved to be hexagonal with little differences in the lattice parameters. The energy dispersive spectrometer and Raman spectra demonstrate a wide range of the x in NiSe2−xTex from nearly 0 to almost 2, which proves that the chemical composition can be readily tunable.

Anomalous Behavior of 2D Janus Excitonic Layers under Extreme Pressures.

Newly discovered 2D Janus transition metal dichalcogenides layers have gained much attention from a theory perspective owing to their unique atomic structure and exotic materials properties, but little to no experimental data are available on these materials. Here, experimental and theoretical studies establish the vibrational and optical behavior of 2D Janus S-W-Se and S-Mo-Se monolayers under high pressures for the first time. Chemical vapor deposition (CVD)-grown classical transition metal dichalcogenides (TMD) monolayers are first transferred onto van der Waals (vdW) mica substrates and converted to 2D Janus sheets by surface plasma technique, and then integrated into a 500 µm size diamond anvil cell for high-pressure studies. The results show that 2D Janus layers do not undergo phase transition up to 15 GPa, and in this pressure regime, their vibrational modes exhibit a nonmonotonic response to the applied pressures (dω/dP). Interestingly, these 2D Janus monolayers exhibit unique blueshift in photoluminescence (PL) upon compression, which is in contrast to many other traditional semiconductor materials. Overall theoretical simulations offer in-depth insights and reveal that the overall optical response is a result of competition between the ab-plane (blueshift) and c-axis (redshift) compression. The overall findings shed the very first light on how 2D Janus monolayers respond under extreme pressures and expand the fundamental understanding of these materials.

Structural, optical and magnetic properties of SnS2 nanoparticles and photo response characteristics of p-Si/n-SnS2 heterojunction diode

As an emanating layered metal dichalcogenides (LMDs), tin disulfide (SnS2) has immense potentials in next generation nanodevices considering their facile synthesis and versatile attributes. Here, we describe the hydrothermal synthesis of SnS2 nanoparticles at different temperatures and growth times and discussed its structural, optical and magnetic characteristics in detail. The SnS2 nanoparticles displayed growth time dependent structural, optical and magnetic properties. The structural analysis displayed hexagonal phase of SnS2 nanoparticles whose thickness increase with duration of growth. The vibrational modes of SnS2 nanoparticles in Raman spectra exhibited an increase in intensity with growth time, indicating an increased layer thickness with growth time. In addition, the optical properties were also affected by the growth time. Moreover, excitation dependent photoluminescence emission from these nanoparticles is observed, which is attributed to the polydispersity of nanoparticles. The realisation of room temperature ferromagnetism makes it a promising material for spintronic applications. Photocurrent properties of the spin coated n-SnS2 on p-silicon were analysed by studying J-V relationships. Tunable optical, electrical and magnetic properties of SnS2 layers could capacitate a great deal of freedom in modelling atomically thin optoelectronic and spintronic devices.

Demonstration of a polariton step potential by local variation of light-matter coupling in a van-der-Waals heterostructure.

The large oscillator strength of excitons in transition metal dichalcogenide layers facilitates the formation of exciton-polariton resonances for monolayers and van-der-Waals heterostructures embedded in optical microcavities. Here, we show, that locally changing the number of layers in a WSe2/hBN/WSe2 van-der-Waals heterostructure embedded in a monolithic, high-quality-factor cavity gives rise to a local variation of the coupling strength. This effect yields a polaritonic stair case potential, which we demonstrate at room temperature. Our result paves the way towards engineering local polaritonic potentials at length scales down to atomically sharp interfaces, based on purely modifying its real part contribution via the coherent light-matter coupling strength g.

Tin disulfide piezoelectric nanogenerators for biomechanical energy harvesting and intelligent human-robot interface applications

Abstract Tin disulfide nanosheets (SnS2 NSs), belonging to a new family of layered metal dichalcogenides, have evoked considerable attention in multidisplinary scientific applications because of their high electron mobility, excellent chemical stability, and wide accessibility to optoelectronic devices. However, despite the prediction of SnS2 to possess excellent piezoelectricity, only very few attempts have been made to investigate the electromechanical characterisitcs of SnS2. Herein, we report the experimental investigation of the intrinsic piezoelectricity of SnS2 NSs, which were synthesized with a facile chemical vapor deposition (CVD) method. The prominent inverse piezoelectricity of SnS2 NSs was measured via piezoresponse force microscpoy (PFM). To further evaluate the piezoelectric performance, SnS2 NSs were integrated into a piezoelectric nanogenerator (PENG) device to display the energy harvesting and active sensing capabilities. Most importantly, the SnS2 PENG device was utilized to explore the synchronous human-robot control of a smart sign language system, which demonstrates great potential for the future applications in human-machine interface and enabling sensing technology.

One-Dimensional Edge Transport in Few-Layer WTe2.

WTe2 is a layered transitional-metal dichalcogenide (TMD) with a number of intriguing topological properties. Recently, WTe2 has been predicted to be a higher-order topological insulator (HOTI) with topologically protected hinge states along the edges. The gapless nature of WTe2 complicates the observation of one-dimensional (1D) topological states in transport due to their small contribution relative to the bulk. Here, we study the behavior of the Josephson effect in magnetic field to distinguish edge from bulk transport. The Josephson effect in few-layer WTe2 reveals 1D states residing on the edges and steps. Moreover, our data demonstrates a combination of Josephson transport properties observed solely in another HOTI—bismuth, including Josephson transport over micrometer distances, extreme robustness in a magnetic field, and nonsinusoidal current-phase relation (CPR). Our observations strongly suggest the topological origin of the 1D states and that few-layer WTe2 is a HOTI.

Activating the Basal Plane of Defective SnS2 Nanosheets for NH3 Gas Sensing

Layered metal dichalcogenides (LMDs) have been acknowledged as having an efficient gas-sensing process because of their tunable electronic structure and active edge sites. However, the full operati...

Automated Assembly of Wafer-Scale 2D TMD Heterostructures of Arbitrary Layer Orientation and Stacking Sequence Using Water Dissoluble Salt Substrates.

We report a novel strategy to assemble wafer-scale two-dimensional (2D) transition metal dichalcogenide (TMD) layers of well-defined components and orientations. We directly grew a variety of 2D TMD layers on "water-dissoluble" single-crystalline salt wafers and precisely delaminated them inside water in a chemically benign manner. This manufacturing strategy enables the automated integration of vertically aligned 2D TMD layers as well as 2D/2D heterolayers of arbitrary stacking orders on exotic substrates insensitive to their kind and shape. Furthermore, the original salt wafers can be recycled for additional growths, confirming high process sustainability and scalability. The generality and versatility of this approach have been demonstrated by developing proof-of-concept "all 2D" devices for diverse yet unconventional applications. This study is believed to shed a light on leveraging opportunities of 2D TMD layers toward achieving large-area mechanically reconfigurable devices of various form factors at the industrially demanded scale.

Charge transfers and charged defects in WSe2/graphene-SiC interfaces

We report on Kelvin probe force microscopy (KPFM) and density functional theory (DFT) investigations of charge transfers in vertical heterojunctions between tungsten diselenide (WSe2) layers and graphene on silicon carbide substrates. The experimental data reveal the existence of an interface dipole, which is shown by DFT to originate from the neutralization of the graphene n-doping by an electron transfer towards the transition metal dichalcogenide (TMD) layer. The relative vacuum level shift probed by KPFM between the TMD and the substrate stays constant when passing from monolayer to bilayer graphene, which confirms that the Schottky-Mott model can be rigorously applied to these interfaces by taking into account the charge transfer from the substrate to the TMD. DFT calculations show that the first TMD layer absorbs almost all the excess charges contained in the graphene, and that the second TMD layer shall not play a significant role in the electrostatics of the system. Negatively charged defect at the TMD edges contribute however to the electrostatic landscape probed by KPFM on both TMD layers.

Remarkable improved photoelectric performance of SnS2 field-effect transistor with Au plasmonic nanostructures

The development of photoelectric devices for high integration and miniaturization in the semiconductor industry can be pushed forward by the thriving research of two-dimensional layered metal dichalcogenides (2D-LMDs). SnS2 nanosheets have an evident photoresponse to both ultraviolet and partial visible light, but only with a fair photoelectric performance limited by their atomic-layer thickness. Here, we report a convenient and simple method to dramatically enhance the electrical and photoelectric performance of the SnS2 flake. By integrating SnS2 with Au plasmonic nanostructures, the photocurrent (Iph) increased by over 20 times. The corresponding responsivity (R), light gain (G), and detectivity (D*) have been improved by ∼2200%, 2200% and 600%, respectively. The responsivity and detectivity of the Au NPs-SnS2 field-effect transistor (FET) at 532 nm are 1125.9 A W−1 and 2.12 × 1011 Jones. Though atomically thin, the hybrid SnS2 photodetector, benefiting from local surface plasmonic resonance, achieves an excellent photoelectric performance that is not usually possible with a pristine SnS2-only device.