Ternary Transition Metal Dichalcogenides Research Articles

Tungsten interior doping engineering induced sulfur vacancies of MoS2 for efficient charge transfer and nonlinear optical performance: Implications for optical limiting devices

Modulating the photoelectric properties of molybdenum disulfide (MoS2) through defect engineering and heterometal doping is crucial for its potential applications in electronic and optoelectronic devices. Herein, a comprehensive overview is provided on the advancements of two-dimensional materials with a ternary structure comprising sulfur (S), molybdenum (Mo), and tungsten (W) in the field of optoelectronic device. A ternary W-P/MoS2 (P: direct current target power) nanomaterial was designed and synthesized using W interior doping engineering induced S vacancies. The experimental results reveal that the introduction of W metal causes lattice distortion in MoS2, leading to the formation of S vacancies within W-P/MoS2. Compared to pure MoS2, W-P/MoS2 with S vacancies demonstrates enhanced reverse saturable absorption and optical limiting. Density functional theory calculations suggest that the S vacancies introduced by W doping in MoS2 introduce defect energy levels, which are believed to be the reason for the improved nonlinear optical (NLO) performance of W-P/MoS2. Furthermore, transient absorption spectroscopy reveals the photophysical model of carrier relaxation and presents an explanation for the optimized NLO properties of W-P/MoS2. This work provides a novel strategy for the design and synthesis of ternary transition metal dichalcogenides and modulating the NLO properties by doping transition metal-mediated vacancies.

Synthesis and Formulation of Ternary Transition Metal Dichalcogenide Alloys for Additive Electronic Manufacturing

Compared with the most investigated graphene, layered transition metal dichalcogenides (TMDs) have received significant attention due to their various chemical compositions and great potential electronic application. In order to tailor and engineer properties of TMDs and therefore enhance their efficiency toward commercial applications, several methods have been employed including reducing dimensionality, creating inter and intra-heterostructures, introducing inter stain, and alloying. Among these, alloying via chemical vapor transport (CVT) is scalable, cost- efficient, and controllable method of tuning TMD bulk crystal properties. Alloying of TMDs can be categorized into several types including metal replacement, dichalcogenide replacement, and both metal and dichalcogenide replacement. In a typical TMD CVT reaction, high-purity precursors are mixed in a desired stochiometric ratio, vacuumed, and sealed in an ampoule to form a closed system. The resulting TMD crystal can then be used for additional processes, e.g., exfoliation to yield 2D TMD nanosheets. Additive electronics manufacturing is a promising technique for the scalable fabrication of electronic devices, including sensors and energy storage devices. For an ink to be compatible for different printing modalities the ink rheology including viscosity, surface tension, and solid requires to be tuned to obtain suitable fluid dynamic parameters for jetting the ink. This work summarizes the development, synthesis, characterization, and formulation of two-dimensional ternary TMDCs ink for aerosol jet printing (AJP) technology. Ball milling assisted liquid exfoliation is utilized for synthesizing ternary TMDs nanomaterials under controlled condition. The resulting nanomaterial is developed into nanomaterial ink compatible with AJP. Detailed analysis of the material characterization and ink properties are required to optimize the fluid dynamics and properties of the ink. After printing the formulated ink using AJP on various substrates, post-printing process techniques is required to be investigated for printed nanomaterials inks to achieve bulk-like performance for the printed structures. Our results highlight the innovations in synthesis and formulation of ternary transition metal dichalcogenide nanomaterial inks for additive manufacturing of electronic devices such as sensors, solar cells, and energy storage devices.

An open database of computed bulk ternary transition metal dichalcogenides

We present a dataset of structural relaxations of bulk ternary transition metal dichalcogenides (TMDs) computed via plane-wave density functional theory (DFT). We examined combinations of up to two chalcogenides with seven transition metals from groups 4–6 in octahedral (1T) or trigonal prismatic (2H) coordination. The full dataset consists of 672 unique stoichiometries, with a total of 50,337 individual configurations generated during structural relaxation. Our motivations for building this dataset are (1) to develop a training set for the generation of machine and deep learning models and (2) to obtain structural minima over a range of stoichiometries to support future electronic analyses. We provide the dataset as individual VASP xml files as well as all configurations encountered during relaxations collated into an ASE database with the corresponding total energy and atomic forces. In this report, we discuss the dataset in more detail and highlight interesting structural and electronic features of the relaxed structures.

MoxTa(1-x)Se2 as saturable absorber for ultrafast photonics

Two-dimension ternary transition metal dichalcogenides have drawn great interests from both the fields of nanotechnology and materials science, due to their extraordinary physical performance, and great potential applications in nanodevice. Here, the applications of MoxTa(1-x)Se2 nanosheets in ultrafast photonics are discussed. The MoxTa(1-x)Se2 nanosheets are prepared by liquid phase exfoliation method. A MoxTa(1-x)Se2-based saturable absorber is fabricated by depositing these nanosheets onto the side surface of d-shaped fiber, which is inserted into a Er-doped fiber laser cavity. By adjusting polarization state and pump power, both harmonic mode-locking and large energy mode-locking operations are realized, respectively. In the former state, a 56th harmonic mode-locking is obtained, which corresponds to a repetition rate of 312.5 MHz. For the later state, the maximum average output power of 12.24 mW and single pulse energy of 2.19 nJ are obtained when the pump power is 500mW. The results demonstrate that MoxTa(1-x)Se2 can be widely used in ultrafast photonics and nonlinear optics.

Photo‐Carrier Lifetime in Binary and Ternary Heterostructures of Transition Metal Dichalcogenides

Binary and ternary transition metal dichalcogenide (TMD) heterostructures, MSe2 and MSeS (where M = Mo or W), are investigated as future building blocks for high‐performance device applications with a tailorable electronic bandgap. For light‐emitting device applications, investigating the decay time of the material is a prerequisite. Herein, time‐resolved photoluminescence (TRPL) of binary and ternary TMD heterostructures is measured. A tri‐exponential function is adopted to fit the TRPL data. Short, medium, and long decay times are <50 ps, ≈400 ps, and >400 ps, respectively. Except for monolayer MoSe2 and MoSeS, only two PL decay time components are observed with a large contribution from short decay time photocarriers (>80%). Bilayer TMDs exhibit a much shorter PL decay time, which could be attributed to the indirect nature of excitons and different dielectric environments, compared with that of monolayer TMDs. Using the center‐of‐mass (CM) method, a PL decay time map is successfully obtained for binary and ternary TMD heterostructures. This allows the qualitative visualization of the decay time distribution within the heterostructure or among other materials. This technique could be applied to assess the decay time of materials used in high‐performance light‐emitting devices.

2D layered MSe2 (M = Hf, Ti and Zr) for compact lasers: nonlinear optical properties and GHz lasing

Abstract Two-dimensional (2D) ternary transition-metal dichalcogenides (TMDCs) are of great research interest because their superior layer-dependent optical modulation properties. In this work, three different kinds of TMDC nanosheets, including hafnium diselenide (HfSe2), titanium diselenide (TiSe2) and zirconium diselenide (ZrSe2), are prepared by liquid phase exfoliation (LPE) technique. The high-quality material properties of these TMDC nanosheets are confirmed by Raman spectroscopy and X-ray diffraction analysis. Furthermore, the bandgap information of five-layer MSe2 has been investigated via utilizing density functional theory. The calculation results exhibit ultra-narrow bandgap structure (lower than 1.1 eV) for all these three materials, indicating that MSe2 is suitable for broadband photonic applications. By applying the fabricated MSe2 as saturable absorbers, high-performance Q-switched mode-locked laser operation has been realized. The laser gain media are Nd:GdVO4 cladding waveguides fabricated by femtosecond laser direct writing. As a result, the pulsed waveguide lasers are able to deliver approximately 6-GHz laser pulses with a signal-to-noise ratio of over 45 dB. The minimum pulse width is determined to be as short as 26 ps. The results demonstrated in this work exhibit the great potential of TMDCs and waveguide structures in applications of pulsed lasers with compact footprints.

Two‐Dimensional MoxW1−xS2 Alloys for Nanogenerators Producing Record Piezo‐Output and Coupled Photodetectors for Self‐Powered UV Sensor

AbstractIn this work a novel sustainable, self‐driven UV photodetection system using ternary transition metal dichalcogenide (TMDC) alloy (MoxW1−xS2) is proposed by coupling a photodetector as a light intensity sensor and a piezoelectric nanogenerator (PENG) as a power source. Crystalline MoxW1−xS2 ternary alloy nanosheets are synthesized via cost effective hydrothermal method to fabricate a UV (365 nm) photodetector with extraordinary high responsivity (≈229 A W‐1). The microstructural, optical properties of different ternary alloy composition, along with their binary counterparts, have been studied. The optimized Mo0.5W0.5S2 ternary alloy nanosheets are used as fillers in PVDF matrix to fabricate flexible self‐polled PENGs, exhibiting piezoelectric open‐circuit output ≈50 V under finger tapping and a record high piezo‐voltage ≈187 V under impact of 12 kPa. By coupling the two devices, the output voltage of the PENG is tuned by the resistance of the photodetector, exhibiting superior voltage sensitivity (≈0.75 V µW‐1 cm‐2) with UV illumination. The impedance matching between the PENG and photodetector has been utilized to demonstrate self‐powered UV sensors by monitoring the on/off states of light‐emitting diode directly. This self‐powered, portable ternary TMDC alloy device is attractive for future real‐time monitoring of UV radiation and smart health sensors for IOT applications.

Ternary Transition Metal Dichalcogenides for High Power Vector Dissipative Soliton Ultrafast Fiber Laser

Abstract2D ternary transition metal dichalcogenides (TMDCs) have been studied widely by researchers from the fields of nanotechnology to materials science because of the extraordinary chemical/physical characteristics, and significant potential in nanoscale device applications. Here, the application of NbxRe(1−x)S2 nanosheets in ultrafast photonics is studied. The few‐layer NbxRe(1−x)S2 nanosheets are fabricated through liquid phase exfoliation method and a NbxRe(1−x)S2‐microfiber device is constructed by depositing these nanosheets onto the tapered region of a microfiber. After incorporating the NbxRe(1−x)S2‐microfiber saturable absorber (SA) into a net positive dispersion Er‐doped fiber (EDF) laser cavity, the generation of stable dissipative soliton pulses with a pulse duration of 1.03 ps proved as polarization‐locked vector solitons upon further study. With pump power growing to 900 mW, the average output power increases to 116.9 mW without pulse splitting. Besides, the pulse width can be compressed to 149.6 fs outside the cavity by using a single‐mode fiber. Compared with previous works based on 2D materials‐based saturable absorbers, the NbxRe(1−x)S2‐based mode‐locked fiber laser proposed herein, reveals superior comprehensive performance.

Stoichiometric modulation on optical nonlinearity of 2D MoS x Se2−x alloys for photonic applications

Abstract The composition-engineered band structures of two-dimensional (2D) ternary transition-metal dichalcogenides (TMDCs) semiconductor alloys directly dominate their electronic and optical properties. Herein, in this paper, a detailed theoretical and experimental study on the composition-dependent nonlinear optical properties of 2D MoS x Se2−x alloys was carried out. The first-principles calculations were performed to investigate the compositionally modulated properties of monolayer 2D MoS x Se2−x (x = 0.25, 0.5, 1.0, 1.5, and 1.75) in terms of the carrier effective mass, carrier density and mobility, as well as band-gaps. Furthermore, high-quality few-layered MoS x Se2−x (x = 0.2, 0.5, 1.0, 1.5, and 1.8) nanosheets were fabricated by using liquid phase exfoliation method. The third-order nonlinear optical response was investigated by open-aperture Z-scan technique, revealing composition-dependent saturable absorption, and light modulation properties, which were correlated to the theoretical calculations and further confirmed by using MoS x Se2−x nanosheets as saturable absorbers (SAs) for all-solid-state pulsed lasers. In particular, a mode-locked solid-state laser with pulse width of 227 fs was realized with MoS0.2Se1.8 as SA, for the first time to our best knowledge. Our work not only provides a comprehensive understanding of the compositionally and defectively modulated nonlinear optical responses of ternary TMDCs alloys, but also paves a way for the development of 2D materials-based novel optoelectronic devices.

Ternary transition metal dichalcogenides for passively Q-switched Er-doped fiber laser applications

A ternary transition metal dichalcogenide based saturable absorber was proposed and employed to generate the Q-switching pulses in a fiber laser. The fabricated MoxTa1−xSe2-based saturable absorber exhibits excellent nonlinear optical absorption properties with saturable intensity of 19.25 MW/cm2 and modulation depth of 19.89%. After incorporating the MoxTa1−xSe2-based saturable absorber into a fiber laser cavity, this fiber laser worked at passively Q-switched state. Moreover, by enhancing the pump power up to 900 mW, the output power was enhanced to 11.86 mW and the pulse width was reduced down to 2.2 μs. The repetition frequency was adjustable in the extent of 94.15–196.31 kHz, and the signal-to-noise ratio was as large as 73 dB. Our results indicate that the proposed MoxTa1−xSe2 materials possess remarkable nonlinear optical characteristics that can be considered as a powerful candidate for pulsed lasers applications.

Heterogeneous Electronic and Photonic Devices Based on Monolayer Ternary Telluride Core/Shell Structures.

Device engineering based on the tunable electronic properties of ternary transition metal dichalcogenides has recently gained widespread research interest. In this work, monolayer ternary telluride core/shell structures are synthesized using a one-step chemical vapor deposition process with rapid cooling. The core region is the tellurium-rich WSe2-2 x Te2 x alloy, while the shell is the tellurium-poor WSe2-2 y Te2 y alloy. The bandgap of the material is ≈1.45eV in the core region and ≈1.57eV in the shell region. The lateral gradient of the bandgap across the monolayer heterostructure allows for the fabrication of heterogeneous transistors and photodetectors. The difference in work function between the core and shell regions leads to a built-in electric field at the heterojunction. As a result, heterogeneous transistors demonstrate a unidirectional conduction and strong photovoltaic effect. The bandgap gradient and high mobility of the ternary telluride core/shell structures provide a unique material platform for novel electronic and photonic devices.

In-situ surface self-reconstruction in ternary transition metal dichalcogenide nanorod arrays enables efficient electrocatalytic oxygen evolution

Water splitting has received more and more attention because of its huge potential to generate clean and renewable energy. The highly active and durable oxygen evolution reaction (OER) catalysts play a decisive factor in achieving efficient water splitting. The identification of authentic active origin under the service conditions can prompt a more reasonable design of catalysts together with well-confined micro-/nano-structures to boost the efficiency of water splitting. Herein, Fe, Co, and Ni ternary transition metal dichalcogenide (FCND) nanorod arrays on Ni foam are purposely designed as an active and stable low-cost OER pre-catalyst for the electrolysis of water in alkaline media. The optimized FCND catalyst demonstrated a lower overpotential than the binary and unary counterparts, and a 27-fold rise in kinetic current density at the overpotential of 300 mV compared to the nickel dichalcogenide counterpart. Raman spectra and other structural characterizations at different potentials reveal that the in-situ surface self-reconstruction from FCND to ternary transition metal oxyhydroxides (FCNOH) on catalyst surfaces initiated at about 1.5 V, which is identified as the origin of OER activity. The surface self-reconstruction towards FCNOH also enables excellent stability, without fading upon the test for 50 h.

Characterization and Stability of Janus TiXY (X/Y = S, Se, and Te) Monolayers

The realization of Janus MoSSe monolayers has brought two-dimensional (2D), ternary transition metal dichalcogenides (TMDs) into focus. The addition of a third element can lead to superior properti...

Mode-locked Tm-doped fiber laser with large modulation depth ReS1.02Se0.98 nanosheet saturable absorber

In this paper, we demonstrate excellent ultrafast optical performance of ternary transition metal dichalcogenide ReS1.02Se0.98 saturable absorber with high power tolerance and large modulation depth in thulium-doped fiber laser. Q-switching with tunable repetition frequency and pulse duration, and mode-locking with high pulse energy were generated in our experiment. The average output power/pulse energy/pulse duration of the mode-locked pulse train were 313 mW/13.6 nJ/1.43 ps, respectively.

Observation of Topological Edge States at the Step Edges on the Surface of Type-II Weyl Semimetal TaIrTe4.

Topological materials harbor topologically protected boundary states. Recently, TaIrTe4, a ternary transition-metal dichalcogenide, was identified as a type-II Weyl semimetal with the minimal nonzero number of Weyl points allowed for a time-reversal invariant Weyl semimetal. Monolayer TaIrTe4 was proposed to host topological edge states, which, however, lacks of experimental evidence. Here, we report on the topological edge states localized at the monolayer step edges of the type-II Weyl semimetal TaIrTe4 using scanning tunneling microscopy. One-dimensional electronic states that show substantial robustness against the edge irregularity are observed at the step edges. Theoretical calculations substantiate the topologically nontrivial nature of the edge states and their robustness against the edge termination and layer stacking. The observation of topological edge states at the step edges of TaIrTe4 surfaces suggests that monolayer TaIrTe4 is a two-dimensional topological insulator, providing TaIrTe4 as a promising material for topological physics and devices.

Tunable and anisotropic photoresponse of layered Re0.2Sn0.8Se2 ternary alloy

In order to produce high-performance optoelectronic devices, alloy engineering has been exploited in ternary transition metal dichalcogenides. In the present study, we demonstrated the effect of rhenium incorporation on the structural and electrical response in SnSe2 layered crystals grown by direct vapour transport technique. Elemental conformation was characterized by EDAX. The powder X-ray diffraction shows the hexagonal phase and high crystallinity of grown compounds, which is also confirmed by the TEM SAED pattern. Perfectly hexagonal shaped microcrystals are shown in SEM surface micrograph. Raman spectra indicate the redshift in the vibrational mode peaks of SnSe2 with the rhenium doping. Current-time characteristic was measured to study to the effect of alloy engineering on the photoresponse ability of the grown compound. Among all, Re0.2Sn0.8Se2 ternary alloy shows the enhanced photoresponse under polychromatic illumination, showing the suppression of deep-level defect states after incorporation of Re in SnSe2 lattice structure. These materials have a layered structure and exhibit the anisotropic conductivity. Hence the anisotropic photodetection characteristics of Re0.2Sn0.8Se2 ternary alloy based photodetectors are further studied in detail. Photocurrent and photo responsivity are higher along the symmetric contacts, whereas along the asymmetric contacts both the quantities are lower in magnitude because the photogenerated carriers have to cross the perpendicular resistance of is layers stacked together by the weak Vander Waal's forces. Increasing the bias voltage enhances the photoresponsivity from 5.141 mAW−1 to 67.231 mAW−1 and photodetectivity 0.971 to 3.452 × 109 Jones. Anisotropic photoresponse was also studied under the monochromatic light. Results provide the significant understanding of tunable and layered structure dependent anisotropic photoresponse ability of TMDC materials.

Ternary ReS2(1-x)Se2x alloy saturable absorber for passively Q-switched and mode-locked erbium-doped all-fiber lasers

We report Q-switched and mode-locked erbium-doped all-fiber lasers using ternary ReS2(1−x)Se2x as saturable absorbers (SAs). The modulation depth and saturable intensity of the film SA are 1.8% and 0.046 MW/cm2. In Q-switched mechanism output, the pulse was centered at 1531.1 nm with maximum pulse energy and minimum pulse width of 28.29 nJ and 1.07 μs, respectively. In mode-locked operation, the pulse was centered at 1561.15 nm with pulse width of 888 fs, repetition rate of 2.95 MHz, and maximum pulse energy of 0.275 nJ. To the best of our knowledge, this is the first report on the mode-locked Er3+-doped fiber laser using ternary transition metal dichalcogenides. This work suggests prospective 2D-material SAs can be widely used in versatile fields due to their attractive optoelectronic and tunable energy bandgap properties.

Ternary MoWSe[formula omitted] alloy saturable absorber for passively Q-switched Yb-, Er- and Tm-doped fiber laser

A ternary molybdenum tungsten selenide (MoWSe2) alloy is proposed and demonstrated as a saturable absorber (SA) to induce Q-switching in the 1. 0μm, 1. 5μm and 2.0μm regions. The MoWSe2 based SA is fabricated by mechanical exfoliation and sandwiched between two fiber ferrules to complete the fiberized SA assembly. The MoWSe2-SA is integrated into ytterbium, erbium and thulium-doped fiber laser cavities, and stable self-starting Q-switched pulses are observed at 1038 nm, 1554 nm and 1964 nm respectively. Maximum repetition rates at 120.2 kHz, 48 kHz and 61.5 kHz are observed at these wavelengths, along with corresponding pulse widths as narrow as 0.96 μs, 1.9 μs and 2.4 μs respectively. These results validate the performance of MoWSe2 as a broadband SA for Q-switching operation, offering new opportunities of ternary transition metal dichalcogenide alloys in future photonics devices.

(Invited) Theory and Device Concepts of Novel Electronic, Optoelectronic, and Topological 2D Materials

2D materials provide extraordinary materials platform with many interesting physical properties distinct from their bulk counterpart and offer unprecedented opportunities for ultrathin electronic and optical applications. Here I will present our recent theoretical works on novel 2D multiferroics and 2D topological materials as well as a few device concepts. First, I will highlight our discoveries on 2D topological materials including binary and ternary transition metal dichalcogenides etc. Our works show that electric field, elastic strain, and van der Waals stacking can be utilized to introduce topological phase transition [1,2]. In particular, the electric field induced topological phase transition offers a possibility for realizing topological field effect transistor, which will be fundamentally different from conventional transistors. In the second example, I will introduce the discovery of 2D multiferroics in semiconducting group IV monochalcogenide monolayers [3]. These 2D materials exhibit giant spontaneous ferroelastic lattice strain and ferroelectric polarization. Both are strongly coupled, accompanied with large anisotropic 2D excitonic responses and nonlinear optical responses [4]. Our theoretical findings indicate that these multiferroic 2D materials possess low domain wall energy and migration barrier, suggesting great opportunities for multifunctional opto-electronic-mechanical devices such as 2D ferroelectric and ferroelastic memory and 2D ferroelectric excitonic photovoltaics. Finally, relevant experimental progresses as well as some critical challenges and opportunities in 2D materials and their nanostructures will be discussed. 1. Xiaofeng Qian, Junwei Liu, Liang Fu, and Ju Li. Quantum spin Hall effect in two-dimensional transition metal dichalcogenides. Science 346, 1344-1347 (2014). 2. Junwei Liu, Hua Wang, Chen Fang, Liang Fu, and Xiaofeng Qian. van der Waals Stacking-Induced Topological Phase Transition in Layered Ternary Transition Metal Chalcogenides. Nano Letters 17, 467-475 (2017). 3. Hua Wang and Xiaofeng Qian. Two-dimensional multiferroics in monolayer group IV monochalcogenides. 2D Materials 4, 015042 (2017). 4. Hua Wang and Xiaofeng Qian. Giant Optical Second Harmonic Generation in Two-Dimensional Multiferroics. Nano Letters 17, 5027-5034 (2017).

Carbon Dioxide Electroreduction into Syngas Boosted by a Partially Delocalized Charge in Molybdenum Sulfide Selenide Alloy Monolayers

AbstractStructural parameters of ternary transition‐metal dichalcogenide (TMD) alloy usually obey Vegard law well, while interestingly it often exhibits boosted electrocatalytic performances relative to its two pristine binary TMDs. To unveil the underlying reasons, we propose an ideal model of ternary TMDs alloy monolayer. As a prototype, MoSeS alloy monolayers are successfully synthesized, in which X‐ray absorption fine structure spectroscopy manifests their shortened Mo−S and lengthened Mo−Se bonds, helping to tailor the d‐band electronic structure of Mo atoms. Density functional theory calculations illustrate an increased density of states near their conduction band edge, which ensures faster electron transfer confirmed by their lower work function and smaller charge‐transfer resistance. Energy calculations show the off‐center charge around Mo atoms not only benefits for stabilizing COOH* intermediate confirmed by its most negative formation energy, but also facilitates the rate‐limiting CO desorption step verified by CO temperature programmed desorption and electro‐stripping tests. As a result, MoSeS alloy monolayers attain the highest 45.2 % Faradaic efficiency for CO production, much larger than that of MoS2 monolayers (16.6 %) and MoSe2 monolayers (30.5 %) at −1.15 V vs. RHE. This work discloses how the partially delocalized charge in ternary TMDs alloys accelerates electrocatalytic performances at atomic level, opening new horizons for manipulating CO2 electroreduction properties.