Two-dimensional Transition Metal Dichalcogenide Materials Research Articles

Titanium Self-Intercalation Induced Formation of Orthogonal (1 × 1) Edge/Surface Reconstruction in 1T-TiSe2: Atomic Scale Dynamics and Mechanistic Study.

Edges and surfaces play indispensable roles in affecting the chemical-physical properties of materials, particularly in two-dimensional transition metal dichalcogenides (TMDCs) with reduced dimensionality. Herein, we report a novel edge/surface structure in multilayer 1T-TiSe2, i.e., the orthogonal (1 × 1) reconstruction, induced by the self-intercalation of Ti atoms into interlayer octahedral sites of the host TiSe2 at elevated temperature. Formation dynamics of the reconstructed edge/surface are captured at the atomic level by in situ scanning transmission electron microscopy (STEM) and further validated by density functional theory (DFT), which enables the proposal of the nucleation mechanism and two growth routes (zigzag and armchair). Via STEM-electron energy loss spectroscopy (STEM-EELS), a chemical shift of 0.6 eV in Ti L3,2 is observed in the reconstructed edge/surface, which is attributed to the change of the coordination number and lattice distortion. The present work provides insights to tailor the atomic/electronic structures and properties of 2D TMDC materials.

Rapid and large-scale synthesis of MoS2via ultraviolet laser-assisted technology for photodetector applications.

Two-dimensional transition metal dichalcogenide (TMDC) thin films have been extensively employed in microelectronics research. Molybdenum disulfide (MoS2), as one of prominent candidates of this class, has been applied in photodetectors, integrated electronic devices, gas sensing, and electrochemical catalysis, owing to its extraordinary optoelectronic, chemical, and mechanical properties. Synthesis of MoS2crystal film is the key to its application. However, the reported technology revealed several drawbacks, containing limited surface area, prolonged high-temperature environment, and unsatisfying crystallinity. In order to enhance the convenience of MoS2applications, there is a pressing need for optimized fabrication technology, which could be quicker, with a large area, with adequate crystallinity and heat-saving. In this work, we presented an ultraviolet laser-assisted synthesis technology, accomplishing rapid growth (with the growth rate of about 40 um s-1) of centimeter-scale MoS2films at room temperature. To achieve this, we self-assembled a displaceable reaction chamber system, coupled with krypton fluoride ultraviolet pulse laser. The laser motion speed and trajectory could be customized in the software, allowing the maskless patterning of crystal films. As application, we exhibited a photodetector with the integration of synthesized MoS2and lead sulfide colloidal quantum dots (PbS CQDs), displaying broadband photodetection from ultraviolet, visible to near-infrared spectrum (365 ~ 1550 nm), with the detectivity of 109~ 1010Jones, and the rising time of 0.2 ~ 0.3 s. This work not only demonstrated a high-process-efficiency synthesis of TMDC materials, but also has opened up new opportunities for ultraviolet laser used in optoelectronics.

Large-scale and stacked transfer of bilayers MoS2 devices on a flexible polyimide substrate

Two-dimensional transition metal dichalcogenides (TMDs), as flexible and stretchable materials, have attracted considerable attention in the field of novel flexible electronics due to their excellent mechanical, optical, and electronic properties. Among the various TMD materials, atomically thin MoS2 has become the most widely used material due to its advantageous properties, such as its adjustable bandgap, excellent performance, and ease of preparation. In this work, we demonstrated the practicality of a stacked wafer-scale two-layer MoS2 film obtained by transferring multiple single-layer films grown using chemical vapor deposition. The MoS2 field-effect transistor cell had a top-gated device structure with a (PI) film as the substrate, which exhibited a high on/off ratio (108), large average mobility (∼8.56 cm2 V−1 s−1), and exceptional uniformity. Furthermore, a range of flexible integrated logic devices, including inverters, NOR gates, and NAND gates, were successfully implemented via traditional lithography. These results highlight the immense potential of TMD materials, particularly MoS2, in enabling advanced flexible electronic and optoelectronic devices, which pave the way for transformative applications in future-generation electronics.

Probing the Electron Transfer Behavior from Excited State Organometal Lead Halide Perovskite Nanocrystals to Molybdenum Disulfide Nanoflowers

The charge transfer study from the excited state of MAPbBr3 perovskite nanocrystals (PNCs) to molybdenum disulfide (MoS2) in chloroform solution is conducted using a fluorescence approach. Due to their exceptional properties and distinctive structures, transition metal dichalcogenides (TMDs) are promising candidates for optoelectronic devices. In this study, we have combined hybrid organic–inorganic perovskite nanocrystals MAPbBr3 with MoS2 solution. A significant electron transfer from MAPbBr3 to MoS2 has been depicted by photoluminescence (PL) spectroscopy, time-correlated single-photon count (TCSPC), transient absorption spectroscopy (TAS), and shortening of the absolute PL quantum yield (PLQY) of hybrid MAPbBr3/MoS2 nanocomposites. Fluorescence quenching of perovskite nanocrystals has shown an increase in quenching efficiency. The photoexcited charge transfer from MAPbBr3 to adjacent MoS2 is further correlated using electrochemical tests that compare reduction and oxidation potential. The results revealed that MoS2 extracts the charge from the excited state of PNCs. This work demonstrates the charge transfer properties of hybrid halide perovskites with two-dimensional TMD materials. A combination of these materials could help us to understand the actual potency of MAPbBr3 PNCs in their real applications.

Imaging of Antiferroelectric Dark Modes in an Inverted Plasmonic Lattice.

Plasmonic lattice nanostructures are of technological interest because of their capacity to manipulate light below the diffraction limit. Here, we present a detailed study of dark and bright modes in the visible and near-infrared energy regime of an inverted plasmonic honeycomb lattice by a combination of Au+ focused ion beam lithography with nanometric resolution, optical and electron spectroscopy, and finite-difference time-domain simulations. The lattice consists of slits carved in a gold thin film, exhibiting hotspots and a set of bright and dark modes. We proposed that some of the dark modes detected by electron energy-loss spectroscopy are caused by antiferroelectric arrangements of the slit polarizations with two times the size of the hexagonal unit cell. The plasmonic resonances take place within the 0.5-2 eV energy range, indicating that they could be suitable for a synergistic coupling with excitons in two-dimensional transition metal dichalcogenides materials or for designing nanoscale sensing platforms based on near-field enhancement over a metallic surface.

Trends in CO2 Reduction on Transition Metal Dichalcogenide Edges

Electrocatalytic CO2 reduction is a promising solution to close the anthropogenic carbon cycle. However, linear scaling relations among reaction intermediates of the CO2 reduction reaction (CO2RR) and the competing hydrogen evolution reaction (HER) limits the Faradaic efficiencies and selectivities of traditional transition metal catalysts. The two-dimensional transition metal dichalcogenides (TMDCs) is a promising class of catalysts for the reaction. Selected transition metal dichalcogenides have been shown experimentally to effectively reduce CO2 and in some cases produce “beyond CO” products. The majority of theoretical investigations have focused on MoS2 and WS2, materials that have been widely studied for various applications, e.g desulfurization, HER, Li metal batteries and optical devices. In this computational study, we go beyond Mo and W and study the edge configurations, adsorption properties and aqueous stability of a range of TMDC materials in the 2H phase. We show that the most stable edge configuration is highly dependent on the specific composition of the material, while adsorption properties of H, CO, and COOH vary significantly with edge configuration. We furthermore find that, while a linear scaling among H and CO adsorbates is observed within each group of the sulfides, selenides, and tellurides, respectively, the scaling is shifted toward weaker H bonding and stronger CO bonding going from sulfides to selenides to tellurides. A similar trend is observed for CO/COOH for some edge configurations. The results indicate that the transition metal ditellurides are a promising class of materials with the potential of reducing CO2 further than CO while limiting HER.

Phonon Scattering in Monolayer Molybdenum Disulfide under Different Defect Concentrations Based on Temperature-Dependent Raman Spectra

Monolayer molybdenum disulfide (MoS2), among other two-dimensional transition-metal dichalcogenides materials, is widely used in a broad range of industries due to its extraordinarily different material properties compared to its bulk counterpart. However, such unique behavior may be greatly affected by its capacity of energy dissipation or heat conduction, largely attributed to its inherent phonon scattering properties. In addition, the phonon properties of MoS2 may be greatly affected by parameters such as temperature, defect concentration, etc., reflected by the Raman spectra evolution of A1g or E2g peaks. In this light, we analyze the combined influences of temperature and defect concentration on phonon scattering for the first time. We specifically elaborate experiments based on the temperature-dependent Raman spectroscopy in order to characterize the effect of defects on phonon scattering properties of MoS2. On this basis, a predictive model is developed for the estimation of phonon lifetime under different defect concentrations that may be served as a brief yet accurate and efficient designer tool in the early stage of defect/phonon engineering. In addition, our study may provide more physical insights toward a comprehensive understanding of the phonon behavior of MoS2, thus paving a way for more practical application potentials enabled by low-dimensional materials.

Novel Design of 0D Nanoparticles-2D Transition-Metal Dichalcogenides Heterostructured Devices for High-Performance Optical and Gas-Sensing Applications

Two-dimensional Transition metal dichalcogenides (TMDCs), have now attracted much attention due to their unique layered structure and physical properties. Up to date, several studies have demonstrated monolayered and few-layered TMDC-based photodetectors with good stability, photo-switching time and broadband detectivity from UV to infrared light region. However, the reported responsivity is not as high as the theoretical expectation, indicating that the light absorption is limited by the atomic thickness of 2D-TMDCs and could still be improved. To overcome the drawback of low absorption in 2D TMDC materials, previous reports have revealed several strategies to enhance the electric field and light-harvesting in these atomically thin TMDC layers by hybridizing plasmonic noble-metal nanoparticles, such as Pt, Au and Ag, to facilitate the light-matter interaction at the surface of semiconductors. In this regard, we aim to combine highly absorptive CuInS2(CIS) nanocrystals with noble metal nanoparticles as the photosensitizer to enhance the intrinsic absorptivity and promote the performance of MoS2-based photodetectors. The interests of noble nanocrystals such as platinum and gold are featured for their distinctive properties of the carrier transportation and the storage when combined with semiconductor materials. The strategy described here acts as a perspective to significantly improve the performance of MoS2-based photodetectors with outstanding detection responsivity with selectable wavelengths by further controlling the size and material of the decorated CIS nanocrystals.In addition to optical sensing, TMDCs have also been developed as a promising candidate for gas-molecule detection. Different from commercial metal oxide gas sensors, TMDCs as sensing materials can be operated at room temperature with good performance, increasing its reliability for future industrial applications. Nevertheless, the relatively low response and long response/recovery time are the main drawbacks of these promising devices. Therefore, we proposed the approach to successfully increase the surface area of TMDCs by a one-step synthesis from WO3 into three-dimensional (3D) WS2 nanowalls through a rapid heating and rapid cooling process. Moreover, the combination of CdS/ZnS or CdSe/ZnS core/shell quantum dots (QDs) with different emission wavelengths and WS2 nanowalls will further improve the performance of WS2-based photodetector devices, including 3.5~4.7 times photocurrent enhancement and shorter response time. The remarkable results of the QD-WS2 hybrid devices to the high non-radiative energy transfer (NRET) efficiency between QDs and our nanostructured material are caused by the spectral overlap between the emission of QDs as the donors and the absorption of WS2 as the acceptors. Additionally, the outstanding NO2 gas-sensing properties of QDs/WS2 devices were demonstrated with a remarkably low detection limit down to 50 ppb with a fast response time of 26.8 s, contributed by tremendous local p-n junctions generated from p-type WS2 nanowalls and n-type CdSe-ZnS QDs in this hybrid system.Our strategies to combine 0D nanoparticles or quantum dots and 2D TMDC materials can significantly enhance the optical sensing and gas molecule sensing properties compared to pristine TMDC-based devices, resulting from the efficient charge or energy transfer between the multi-dimension material system and the creation of local p-n junctions. Moreover, the scalability of these hybrid nanostructures allows our devices to exhibit much more possibilities in advanced multifunctional applications.

Surface plasma–induced tunable nitrogen doping through precursors provides 1T-2H MoSe2/graphene sheet composites as electrocatalysts for the hydrogen evolution reaction

In this paper we present that the surface plasma-induced tunable nitrogen doping that requires short processing time at low temperature through various nitrogen precursors that converted the 2H phase into a coexisting 1T-2H phase in the MoSe2 nanosheets; subsequently combining the N-doped MoSe2 with graphene sheets (G) for synthesis of N-doped 1T-2H MoSe2/G composites (N-MoSe2/G) for the hydrogen evolution reaction (HER). We found that the amount of the 1T phase in the MoSe2 nanosheets can be tuned by selecting suitable N-atom precursors—such as NH4NO3, urea and melamine—with our method; moreover, the amount of the 1T phase of the composites induced increases with nitrogen doping concentration at low concentration range (Ca. 4.7%—7.3%) but decreased when the nitrogen doping is over 7.3%. Subsequently, after covering the N-MoSe2 with graphene sheets, the composites featuring highly active sites that facilitated the hydrogen evolution reaction (HER). As a result, the N-MoSe2/G composites exhibited a low overpotential of 153 mV at a current density of 10 mA/cm2 and a Tafel slope of 67 mV/dec, much better than those of undoped-MoSe2 (252 mV, 123 mV/dec) and the N-MoSe2 nanosheets (197 mV, 83 mV/dec). The N-MoSe2/G composites also functioned as a stable electrocatalyst, undergoing over 10,000 cycles of linear sweep voltammetry without degradation. Our approach to tunable nitrogen doping in MoSe2 nanosheets and forming composites provides a feasible way to produce electrocatalysts in mass-scale and low energy process with two-dimensional transition metal dichalcogenide materials.

Electrostatically Controllable Channel Thickness and Tunable Low-Frequency Noise Characteristics of Double-Gated Multilayer MoS2 Field-Effect Transistors with h-BN Dielectric.

Two-dimensional transition-metal dichalcogenide (TMD) materials have attracted increasing attention in efforts to overcome fundamental issues faced by the complementary metal-oxide-semiconductor industry. Multilayer TMD materials such as MoS2 can be used for high-performance transistor-based applications; the drive currents are high and the materials handle low-frequency (LF) noise well. We fabricated double-gated multilayer MoS2 transistors using the h-BN dielectric for the top gate and silicon dioxide for the bottom gate. We systemically investigated the bottom gate voltage (Vb)-controlled electrical characteristics and the top/bottom interface-coupling effects. The effective thickness of the MoS2 channel (tMoS2_eff) was well modulated by Vb, and tMoS2_eff reduction by negative Vb dramatically improved the Ion/Ioff ratio. Numerical simulation and analytical modeling with a variation of the depletion depth under different bias conditions verified the experimental results. We were also the first to observe Vb-tuned LF noise characteristics. Here, we discuss the Vb-affected series resistance and carrier mobility in detail. Our findings greatly enhance the understanding of how double-gated multilayer MoS2 transistors operate and will facilitate performance optimization in the real world.

Resonance Photoluminescence Enhancement of Monolayer MoS2 via a Plasmonic Nanowire Dimer Optical Antenna.

Two-dimensional transition-metal dichalcogenides (TMDs) such as monolayer MoS2 exhibit remarkable optical properties. However, the intrinsic absorption and emission rates of MoS2 are very low, thus severely hindering its application in electronics and photonics. Combining MoS2 with a plasmonic optical antenna is an alternative solution to enhance the emission rates of the 2D semiconductor, and this can drastically increase the photoresponsivity of the corresponding photodetector. Herein, we have constructed a plasmonic gap cavity of a nanowire dimer (NWD) system as an optical antenna to brighten the emission of MoS2 off the hot spot. Different from the conventional enhancement concept which occurred in the plasmonic hot spot, the light emission off the nanogap hot spot was thoroughly investigated. We demonstrate that this new plasmonic optical nanostructure leads to a strong enhancement due to the Purcell effect. The NWD optical antenna can trap light to the near field through a high-efficiency plasmonic gap mode (PGM); then the PL emission was enhanced drastically up to 14.5-fold due to the resonance of the plasmonic gap mode (PGM) in the NWD with the excitonic band of monolayer MoS2. Theoretical simulations reveal that this NWD can alter the efficiency of convergence and excitation, which was consistent with our experimental results. This study can provide a pathway toward enhancing and controlling PGM-enhanced light emission of TMD materials beyond the plasmonic hot spot.

Interface-Induced Seebeck Effect in PtSe2/PtSe2 van der Waals Homostructures.

The Seebeck effect refers to the production of an electric voltage when different temperatures are applied on a conductor, and the corresponding voltage-production efficiency is represented by the Seebeck coefficient. We report a Seebeck effect: thermal generation of driving voltage from the heat flowing in a thin PtSe2/PtSe2 van der Waals homostructure at the interface. We refer to the effect as the interface-induced Seebeck effect. By exploiting this effect by directly attaching multilayered PtSe2 over high-resistance PtSe2 thin films as a hybridized single structure, we obtained the highly challenging in-plane Seebeck coefficient of the PtSe2 films that exhibit extremely high resistances. This direct attachment further enhanced the in-plane thermal Seebeck coefficients of the PtSe2/PtSe2 van der Waals homostructure on sapphire substrates. Consequently, we successfully enhanced the in-plane Seebeck coefficients for the PtSe2 (10 nm)/PtSe2 (2 nm) homostructure approximately 42% compared to that of a pure PtSe2 (10 nm) layer at 300 K. These findings represent a significant achievement in understanding the interface-induced Seebeck effect and provide an effective strategy for promising large-area thermoelectric energy harvesting devices using two-dimensional transition metal dichalcogenide materials, which are ideal thermoelectric platforms with high figures of merit.

Chemical vapor deposited WS2/MoS2 heterostructure photodetector with enhanced photoresponsivity

Two-dimensional transition metal dichalcogenides (TMDCs) have attracted great interest due to their unique semiconductor properties. Among all TMDC materials, MoS2 and WS2 are promising for composing heterostructures. However, traditional TMDC heterostructure fabrication depends on transfer process, with drawbacks of interface impurity and small size. In this work, a two-step chemical vapor deposition (CVD) process was applied to synthesize large-scale WS2/MoS2 heterostructure. Surface morphology and crystal structure characterizations demonstrate the high-quality WS2/MoS2 heterostructure. The WS2/MoS2 heterostructure photodetector fabricated by photolithography exhibits an enhanced photoresponsivity up to 370 A W−1 in comparison with single WS2 or MoS2 devices. This study suggests a direct CVD growth of large-scale TMDC heterostructure films with clean interface. The built-in electric field at interface contributes to the separation of photo-generated electron–hole pairs, leading to enhanced photocurrent and responsivity, and showing promising potentials in photo-electric applications.

Ferromagnetic and nonmagnetic 1T′ charge density wave states in transition metal dichalcogenides: Physical mechanisms and charge doping induced reversible transition

The charge density wave (CDW) states of two-dimensional transition metal dichalcogenides (TMDs) originate from intrinsic couplings between the electronic structures and lattice distortion, inducing interesting physical and chemical properties. The observed TMDs CDW states are mostly nonmagnetic (NM) but with a few ferromagnetic (FM) cases. Physical mechanisms for the formation of FM CDW remain elusive. In this paper, we used density functional theory calculations to study a set of TMDs with magnetic transition metal elements (e.g., V, Cr, and Mn). We found that the FM state can stem from the direct exchange to superexchange transition (e.g., $\mathrm{Cr}{X}_{2}$) or the $M\text{\ensuremath{-}}M$ ($M$ is the metal atom) dimerization (e.g., $\mathrm{Mn}{X}_{2}$). A crystal structure distortion index is proposed to distinguish the different formation mechanisms of FM CDW states. Interestingly, $\mathrm{Cr}{X}_{2}$ has both NM and FM CDW states, which is not observed in other TMDs materials. We found that charge (electron or hole) doping could modulate the different formation mechanisms and induce a phase transition between these two CDW states in ${\mathrm{CrS}}_{2}$, leading to significant actuation strain output (i.e., 12.17% and 5.93% along $x$ and $y$ directions, respectively) and drastic change of magnetism, which could enable some multifunctionality applications of TMD materials.

Performance improvement of semi-transparent ultra-thin CIGSe solar cell by transferring exfoliated WTe2 multilayered-2D flakes to ITO substrate

Cu(In1-x,Gax)Se2 (CIGSe)-based solar cells have a high absorption coefficient (∼105), by which they can be used as thin-film solar cells that have the potential to be applied in various fields. To employ such a CIGSe-based solar cell as a transparent type, the conventional Mo substrate should be replaced with a transparent conductive oxide (TCO) substrate as the back electrode, and indium tin oxide (ITO) is one of the candidate TCO materials. However, various problems need to be addressed because the utilization of ITO may hinder solar cell performance, one of which is the contact occurring at the interface between CIGSe and ITO. This can be solved by adding an appropriate material at the interface while preserving the inherent purpose fulfilled by using the ITO electrode. In this study, we attempt to improve the contact properties using semi-metallic WTe2 flakes dispersed on the surface of the ITO substrate to form an electrical path between the CIGSe and the ITO substrate, which exhibit outstanding electrical conductivity among two-dimensional transition metal dichalcogenide materials. Accordingly, the effect of applying WTe2 flakes at the ITO and CIGSe interfaces on the performance of semi-transparent ultra-thin (STUT) CIGSe solar cells is discussed.

Strong coupling between two-dimensional transition metal dichalcogenides and plasmonic-optical hybrid resonators

In the past decade, strong coupling of two-dimensional transition metal dichalcogenides (TMDs) with plasmonic and optical resonators has become an attractive field in cavity quantum electrodynamics. On one hand, how to enhance the Rabi splitting in such a system is important for applications. On the other hand, the underlying strong-coupling mechanism is not clear yet, especially for the complicated coupling case related to multiple photonic quasiparticles. Here we propose a plasmonic-optical hybrid resonator composed of a Au nanoantenna and photonic crystal cavity. When monolayer $\mathrm{Mo}{\mathrm{S}}_{2}$ is coupled with this hybrid resonator, we demonstrate a large Rabi splitting up to 460 meV, which is better than ever reported in similar systems. More importantly, we provide a deeper understanding of complicated coupling mechanisms in such a hybrid system, paving the way for studying strong light--matter interactions with multiple photonic quasiparticles in TMD materials coupled with different plasmonic-optical hybrid resonators.

WSe2 crystals on paper: flexible, large area and broadband photodetectors

The paper-based photodetector has recently captivated a great deal of attention in various opto-electronics applications because of facile, cost effective and green synthesis. Two-dimensional transition metal dichalcogenides materials are promising for photodetection under the broad spectral range. In this work, we have fabricated paper-based device by rubbing the tungsten di-selenide (WSe2) crystals on paper substrate. Low-cost, facile and green synthesis technique was employed to make a high-performance paper-based WSe2 photodetector. Paper-based photodetector was fabricated via non-toxic simply rubbing process of WSe2 nanosheets on low-cost bio-degradable paper. The photodetector shows good responsivity of 72.5 μA W−1 and detectivity at around 2.4 × 107 Jones at very low bias (1.0 V) at wavelength of 780 nm, respectively. Due to good photo-absorption strength, photodetector exhibits excellent photo-response over wide wavelength range from visible to near infrared. This device also shows very good flexibility with a stable photo-response. This device shows a general and reliable study for the design of photodetectors that is eco-friendly and cost-effective. Overall studied results of the fabricated device indicate that they have the ability to be used in large-scale preparation of the device.

Oscillations of the electron energy loss rate in two-dimensional transition-metal dichalcogenides in the presence of a quantizing magnetic field

We study the hot electron energy-loss rate (ELR) induced by acoustic ${P}^{\mathrm{ac}}$ and optical ${P}^{\mathrm{op}}$ phonons, in two-dimensional transition-metal dichalcogenides (TMDCs), in presence of quantizing magnetic field $B$, including the hot-phonon effect. At the low-temperature regime, the ELR is found to display a quantum oscillations with their amplitude being found to increase with the increase of the magnetic field. The unscreened TA phonon due to deformation potential (DP) coupling is dominant over the other screened acoustic phonon contributions. In the extreme Bloch-Gr\"uneisen (BG) region, the ELR displays a pronounced ${P}^{\mathrm{ac}}\ensuremath{\propto}{T}^{4}\phantom{\rule{0.16em}{0ex}}({T}^{6})$ temperature dependence for unscreened TA-DP phonons (other screened mechanisms). When the temperature increases, ${T}_{e}>{T}_{\mathrm{BG}}$, the ELR shows a linear ${P}^{\mathrm{ac}}\ensuremath{\propto}T$ behavior in the equipartition region. At higher temperatures, there is a crossover from ${P}^{\mathrm{ac}}$ dominated ELR to ${P}^{\mathrm{op}}$ by zeroth-order DP coupling dominated ELR with the cross-over temperature being about ${T}_{e}\ensuremath{\sim}50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The hot phonon effect is demonstrated to reduce ELR significantly. The effect of the magnetic field is found to enhance the ELR significantly, making $B$ as its another tuning knob. Among the four TMDC materials, ${\mathrm{MoS}}_{2}$ (${\mathrm{MoSe}}_{2}$) displays the biggest ELR due to acoustic (optical) phonon scattering while ${\mathrm{WSe}}_{2}$ (${\mathrm{WS}}_{2}$) shows the smallest ELR. Our results for the ${\mathrm{MoS}}_{2}$ material are compared with those of the zero-field ELR.

Multifunctional molybdenum disulfide flash memory using a PEDOT:PSS floating gate

Two-dimensional transition metal dichalcogenide materials (TMDs), such as molybdenum disulfide (MoS2), have been considered promising candidates for future electronic applications owing to their electrical, mechanical, and optical properties. Here, we present a new concept for multifunctional MoS2 flash memory by combining a MoS2 channel with a PEDOT:PSS floating layer. The proposed MoS2 memory devices exhibit a switching ratio as high as 2.3 × 107, a large memory window (54.6 ± 7.80 V), and high endurance (>1,000 cycles). As the PEDOT:PSS film enables a low-temperature solution-coating process and mechanical flexibility, the proposed P-memory can be embedded on a polyimide substrate over a rigid silicon substrate, offering high mechanical endurance (over 1,000 cycle bending test). Furthermore, both MoS2 and PEDOT:PSS have a bandgap that is desirable in optoelectronic memory operation, where charge carriers are stored differently in the floating gate depending on light illumination. As a new application that combines photodiodes and memory functions, we demonstrate multilevel memory programming based on light intensity and color.

One-Step Hydrothermal Synthesis of Phase-Engineered MoS2/MoO3 Electrocatalysts for Hydrogen Evolution Reaction

The development of suitable approaches for the synthesis of ultrathin transition-metal dichalcogenide (TMD) catalysts is required to engineer phases, intercoupling between different phases, in-plane defects, and edges and hence maximize their catalytic performance for hydrogen production. In this work, we report a simple one-step hydrothermal approach for the synthesis of a three-dimensional (3D) network of self-assembled metallic MoS2/MoO3 nanosheets, using α-MoO3 and thiourea (TU) as the Mo and S precursors, respectively. A systematic structural/property relationship study, while varying the precursors’ molar concentration ratios (TU/MoO3) and reaction temperatures (TR), revealed a kinetically controlled regime, in hydrothermal synthesis, that enabled the formation of ultrathin branched MoS2/MoO3 nanosheets with the highest metallic content of ∼47 % in a reproducible manner. Importantly, the work established that in addition to the rich metallic MoS2 phase (1T), the electronically coupled interfaces between MoO3 and MoS2 nanodomains, profusion of active sites, and tuned electrical conductivity significantly contributed to hydrogen evolution reaction (HER)-catalytic activity, affording a low overpotential of 210 mV (with respect to the reversible hydrogen electrode) at a current density of 10 mA/cm2, a small Tafel slope of ∼50 mV/dec, and high stability. Overall, this work demonstrated a controllable one-step hydrothermal method for the rational design and synthesis of a 3D network of MoS2/MoO3 nanosheets with high 1T-MoS2 metallic yield, simultaneous incorporation of MoO3/MoS2 heterointerfaces, sulfur vacancies, and tuned electrical conductivity, which are highly beneficial for clean energy conversion applications that can potentially be expanded to other two-dimensional TMD materials.