Ballistic transport in single-layer MoS<Subscript>2</Subscript> piezotronic transistors

Because of the coupling between semiconducting and piezoelectric properties in wurtzite materials, strain-induced piezo-charges can tune the charge transport across the interface or junction, which is referred to as the piezotronic effect. For devices whose dimension is much smaller than the mean free path of carriers (such as a single atomic layer of MoS₂), ballistic transport occurs. In this study, transport in the monolayer MoS₂ piezotronic transistor is studied by presenting analytical solutions for two-dimensional (2D) MoS₂. Furthermore, a numerical simulation for guiding future 2D piezotronic nanodevice design is presented.

Open image in new window

     

Real-time electrical detection of epidermal skin MoS<Subscript>2</Subscript> biosensor for point-of-care diagnostics

Various approaches have been proposed for point-of-care diagnostics,and in particular,optical detection is preferred because it is relatively simple and fast.At the same time,field-effect transistor (FET)-based biosensors have attracted great attention because they can provide highly sensitive and label-free detection.In this work we present highly sensitive,epidermal skin-type point-of-care devices with system-level integration of flexible MoS2 FET biosensors,read-out circuits,and light-emitting diode (LEDs) that enable real-time detection of prostate cancer antigens (PSA).Regardless of the physical forms or mechanical stress conditions,our proposed high-performance MoS2 biosensors can detect a PSA concentration of 1 pg.mL-1 without specific surface treatment for anti-PSA immobilization on the MoS2 surface on which we characterize and confirm physisorption of anti-PSA using Kelvin probe force microscopy (KPFM)and tapping-mode atomic force microscopy (tm-AFM).Furthermore,current modulation induced by the binding process was stably maintained for longer than 2-3 min.The results indicate that flexible MoS2-based FET biosensors have great potential for point-of-care diagnostics for prostate cancer as well as other biomarkers.     

Size-controlled MoS<Subscript>2</Subscript> nanodots supported on reduced graphene oxide for hydrogen evolution reaction and sodium-ion batteries

Transition metal dichalcogenide nanodots (NDs) have received considerable interest.We report a facile bottom-up synthetic route for MoS2 NDs by using molybdenum pentachloride and L-cysteine as precursors in oleylamine.The synthesis of NDs with a narrow size distribution ranging from 2.2 to 5.3 nm,was tailored by controlling the reaction time.Because of its coating characteristics,oleyalmine leads to uniformity and monodispersity of the NDs.Moreover,the NDs synthesized have large specific surface areas providing active sites.Graphene possesses outstanding conductivity.Combining the advantages of the two materials,the 0D/2D material exhibits superior electrochemical performance because of the 2D permeable channels for ion adsorption,energy storage,and conversion.The as-prepared MoS2/rGO (～2.2 nm) showed a stable capacity of 220 mAh·g-1 after 10,000 cycles at the current density of 20 A·g-1.Furthermore,a reversible capacity ～140 mAh·g-1 was obtained at a much higher current density of 40 A·g-1.Additionally,this composite exhibited superior catalytic performance evidenced by a small overpotential (222 mV) to afford 10 mA·cm-2,and a small Tafel slope (59.8 mV·decade-1) with good acid-stability.The facile approach may pave the way for the preparation of NDs with these nanostructures for numerous applications.     

Defect-rich MoS<Subscript>2</Subscript> nanowall catalyst for efficient hydrogen evolution reaction

Designing efficient electrocatalysts for the hydrogen evolution reaction (HER) has attracted substantial attention owing to the urgent demand for clean energy to face the energy crisis and subsequent environmental issues in the near future.Among the large variety of HER catalysts,molybdenum disulfide (MoS2) has been regarded as the most famous catalyst owing to its abundance,low price,high efficiency,and definite catalytic mechanism.In this study,defect-engineered MoS2 nanowall (NW) catalysts with controllable thickness were fabricated and exhibited a significantly enhanced HER performance.Benefiting from the highly exposed active edge sites and the rough surface accompanied by the robust NW structure,the defect-rich MoS2 NW catalyst with an optimized thickness showed an ultralow onset overpotential of 85 mV,a high current density of 310.6 mA·cm-2 at η =300 mV,and a low potential of 95 mV to drive a 10 mA.cm-2 cathodic current.Additionally,excellent electrochemical stability was realized,making this freestanding NW catalyst a promising candidate for practical water splitting and hydrogen production.     

MoS<Subscript>2</Subscript> as a long-life host material for potassium ion intercalation

Electrochemical potassium ion intercalation into two-dimensional layered MoS2 was studied for the first time for potential applications in the anode in potassium-based batteries.X-ray diffraction analysis indicated that an intercalated potassium compound,hexagonal K0.4MoS2,formed during the intercalation process.Despite the size of K+,MoS2 was a long-life host for repetitive potassium ion intercalation and de-intercalation with a capacity retention of 97.5％ after 200 cycles.The diffusion coefficient of the K+ ions in KxMoS2 was calculated based on the Randles-Sevcik equation.A higher K+ intercalation ratio not only encountered a much slower K+ diffusion rate in MoS2,but also induced MoS2 reduction.This study shows that metal dichalcogenides are promising potassium anode materials for emerging K-ion,K-O2,and K-S batteries.     

High-concentration dispersions of exfoliated MoS<Subscript>2</Subscript> sheets stabilized by freeze-dried silk fibroin powder

Liquid-phase exfoliation (LPE) is an attractive method for the scaling-up of exfoliated MoS₂ sheets compared to chemical vapor deposition and mechanical cleavage. However, the MoS₂ nanosheet yield from LPE is too small for practical applications. We report a facile method for the scaling-up of exfoliated MoS₂ nanosheets using freeze-dried silk fibroin powders. Compared to MoS₂ dispersion in the absence of silk fibroin powder, sonicated MoS₂ dispersions with silk fibroin powder (MoS₂/Silk dispersion) show noticeably higher exfoliated MoS₂ nanosheet yields, with suspended MoS₂ concentrations in MoS₂/Silk dispersions sonicated for 2 and 5 h of 1.03 and 1.39 mg·mL^–1, respectively. The MoS₂ concentration in the MoS₂/Silk dispersion after centrifugation above 10,000 rpm is more than four times that without the silk fibroin. The size of the dispersed silk fibroin is controlled by the change of centrifugation rate, showing the removal of silk fibroin above tens of micrometers in size after centrifugation at 2,000 rpm. Size-controlled silk fibroin biomolecules combined with MoS₂ nanosheets are expected to increase the practical use of such materials in fields related to tissue engineering, biosensors and electrochemical electrodes. Atomic force microscopy and Raman spectroscopy provide the height of the MoS₂ nanosheets spin-cast from MoS₂ /Silk dispersions, showing thicknesses of 3–6 nm. X-ray photoelectron spectroscopy and X-ray diffraction indicate that the outermost surface layer of the hydrophobic MoS₂ crystals interact with oxygen-containing functional groups that exist in the hydrophobic part of silk fibroins. The amphiphilic properties of silk fibroin combined with the MoS₂ nanosheets stabilize dispersions by enhancing solvent-material interactions. The large quantities of exfoliated MoS₂ nanosheets suspended in the as-synthesized dispersions can be utilized for the fabrication of vapor and electrochemical devices requiring high MoS₂ nanosheets contents.

Open image in new window

     

Ultra-small MoS<Subscript>2</Subscript> nanodots with rapid body clearance for photothermal cancer therapy

The clinical translation of many inorganic nanomaterials is severely hampered by toxicity issues because of the long-term retention of these nanomaterials in the body. In this study, we developed a bio-clearable theranostic agent based on ultra-small MoS₂ nanodots, which were synthesized by a facile bottom-up approach through one-step solvothermal decomposition of ammonium tetrathiomolybdate. After modification by glutathione (GSH), the obtained MoS₂-GSH nanodots exhibited sub-10-nm hydrodynamic diameters without aggregation in various physiological buffers. Without showing appreciable in vitro toxicity, such MoS₂-GSH nanodots with strong near-infrared (NIR) absorbance could induce remarkable photothermal ablation of cancer cells. Upon intravenous (i.v.) injection, efficient tumor accumulation of MoS₂-GSH nanodots was observed by photoacoustic imaging, and further confirmed by analysis of the biodistribution of Mo. Notably, the MoS₂-GSH nanodots, in contrast to conventional MoS₂ nanoflakes with larger sizes, showed rather efficient body clearance via urine, where the majority of the injected dose was cleared within just seven days. Photothermal ablation of tumors on mice was then realized with the MoS₂-GSH nanodots, achieving excellent therapeutic efficacy. This study presents a new type of ultra-small nanoparticle with efficient tumor homing/treatment abilities, as well as rapid body clearance behavior, making it promising for cancer theranostics without long-term toxicity concerns.

Open image in new window

     

Probing the intrinsic optical quality of CVD grown MoS<Subscript>2</Subscript>

Optical emission efficiency of two-dimensional layered transition metal dichalcogenides (TMDs) is one of the most important parameters affecting their optoelectronic performance.The optimization of the growth parameters by chemical vapor deposition (CVD) to achieve optoelectronic-grade quality TMDs is,therefore,highly desirable.Here,we present a systematic photoluminescence (PL) spectroscopic approach to assess the intrinsic optical and crystalline quality of CVD grown MoS2 (CVD MoS2).We propose the use of the intensity ratio between the PL measured in air and vacuum as an effective way to monitor the intrinsic optical quality of CVD MoS2.Low-temperature PL measurements are also used to evaluate the structural defects in MoS2,via defect-associated bound exciton emission,which well correlates with the field-effect carrier mobility of MoS2 grown at different temperatures.This work therefore provides a sensitive,noninvasive method to characterize the optical properties of TMDs,allowing the tuning of the growth parameters for the development of optoelectronic devices.     

Hierarchical MoS<Subscript>2</Subscript> intercalated clay hybrid nanosheets with enhanced catalytic activity

Emerging hierarchical MoSR/pillared-montmorillonite (MoS2/PMMT) hybrid nanosheets were successfully prepared through facile in-situ hydrothermal synthesis of MoS2 within the interlayer of cetyltrimethylammonium bromide PMMT,and their catalytic performance was evaluated by the reduction reaction of 4-nitrophenol (4-NP) using NaBH4 as a reductant.Microstructure and morphology characterization indicated that MoS2/PMMT exhibited hybrid-stacked layered structures with an interlayer spacing of 1.29 nm,and the MoS2 nanosheets were intercalated within the montmorillonite (MMT) layers,with most of the edges exposed to the outside.The catalytic activity and stability of MoS2/PMMT were both enhanced by the MMT.With the MoS2/PMMT as the catalyst,the apparent reaction rate constant of the 4-NP reduction was 0.723 min-1 and was maintained at ～0.679 min-1 after five reaction cycles.The structural evolution of MoS2/PMMT and the possible catalysis mechanism for the reduction reaction of 4-NP were investigated.The as-prepared MoS2/PMMT hybrid nanosheets are promising candidates for catalytic application in the water-treatment and biomedical fields.The strategy developed in this study can provide insights for designing hybrid nanosheets with diverse heterogeneous two-dimensional (2D) nanomaterials.     

Thermal conduction across the one-dimensional interface between a MoS<Subscript>2</Subscript> monolayer and metal electrode

The thermal conductance across the one-dimensional (1D) interface between a MoS2 monolayer and Au electrode (edge-contact) has been investigated using molecular dynamics simulations.Although the thermal conductivity of monolayer MoS2 is 2-3 orders of magnitude lower than that of graphene,the covalent bonds formed at the interface enable interfacial thermal conductance (ITC) that is comparable to that of a graphene-metal interface.Each covalent bond at the interface serves as an independent channel for thermal conduction,allowing ITC to be tuned linearly by changing the interfacial bond density (controlling S vacandes).In addition,different Au surfaces form different bonding configurations,causing large ITC variations.Interestingly,the S vacancies in the central region of MoS2 only slightly affect the ITC,which can be explained by a mismatch of the phonon vibration spectra.Further,at room temperature,ITC is primarily dominated by phonon transport,and electron-phonon coupling plays a negligible role.These results not only shed light on the phonon transport mechanisms across 1D metal-MoS2 interfaces,but also provide guidelines for the design and optimization of such interfaces for thermal management in MoS2-based electronicdevices.     

Transformation of monolayer MoS<Subscript>2</Subscript> into multiphasic MoTe<Subscript>2</Subscript>: Chalcogen atom-exchange synthesis route

Molybdenum ditelluride (MoTe2),which is an important transition-metal dichalcogenide,has attracted considerable interest owing to its unique properties,such as its small bandgap and large Seebeck coefficient.However,the batch production of monolayer MoTe2 has been rarely reported.In this study,we demonstrate the synthesis of large-domain (edge length exceeding 30 μm),monolayer MoTe2 from chemical vapor deposition-grown monolayer MoS2 using a chalcogen atom-exchange synthesis route.An in-depth investigation of the tellurization process reveals that the substitution of S atoms by Te is prevalently initiated at the edges and grain boundaries of the monolayer MoS2,which differs from the homogeneous selenization of MoS2 flakes with the formation of alloyed Mo-S-Se hybrids.Moreover,we detect a large compressive strain (approximately-10％) in the transformed MoTe2 lattice,which possibly drives the phase transition from 2H to 1T'at the reaction temperature of 500 ℃.This phase change is substantiated by experimental facts and first-principles calculations.This work introduces a novel route for the templated synthesis of two-dimensional layered materials through atom substitutional chemistry and provides a new pathway for engineering the strain and thus the intriguing physics and chemistry.     

Ultrathin Co(Ni)-doped MoS<Subscript>2</Subscript> nanosheets as catalytic promoters enabling efficient solar hydrogen production

The design of efficient artificial photosynthetic systems that harvest solar energy to drive the hydrogen evolution reaction via water reduction is of great importance from both the theoretical and practical viewpoints.Integrating appropriate co-catalyst promoters with strong light absorbing materials represents an ideal strategy to enhance the conversion efficiency of solar energy in hydrogen production.Herein,we report,for the first time,the synthesis of a class of unique hybrid structures consisting of ultrathin Co(Ni)-doped MoS2 nanosheets (co-catalyst promoter) intimately grown on semiconductor CdS nanorods (light absorber).The as-synthesized one-dimensional CdS@doped-MoS2 heterostructures exhibited very high photocatalytic activity (with a quantum yield of 17.3％) and stability towards H2 evolution from the photoreduction of water.Theoretical calculations revealed that Ni doping can increase the number of uncoordinated atoms at the edge sites of MoS2 nanosheets to promote electron transfer across the CdS/MoS2 interfaces as well as hydrogen reduction,leading to an efficient H2 evolution reaction.     

Thickness-dependent morphologies of Ag on <Emphasis Type="Italic">n</Emphasis>-layer MoS<Subscript>2</Subscript> and its surface-enhanced Raman scattering

The size and density of Ag nanoparticles on n-layer MoS₂ exhibit thicknessdependent behavior. The size and density of these particles increased and decreased, respectively, with increasing layer number (n) of n-layer MoS₂. Furthermore, the surface-enhanced Raman scattering (SERS) of Ag on this substrate was observed. The enhancement factor of this scattering varied with the thickness of MoS₂. The mechanisms governing the aforementioned thickness dependences are proposed and discussed.

Open image in new window

     

Unsaturated-sulfur-rich MoS<Subscript>2</Subscript> nanosheets decorated on free-standing SWNT film: Synthesis,characterization and electrocatalytic application

Herein, we report a bottom-up solvothermal route to synthesize a flexible, highly efficient MoS₂@SWNT electrocatalyst for hydrogen evolution reactions (HER). Characterization revealed that branch-like MoS₂ nanosheets containing sulfurrich sites were in situ uniformly dispersed on free-standing single-walled carbon nanotube (SWNT) film, which could expose more unsaturated sulfur atoms, allowing excellent electrical contact with active sites. The flexible catalyst exhibited excellent HER performance with a low overpotential (~150 mV at 10 mA/cm²) and small Tafel slope (41 mV/dec). To further explain the improved performance, the local electronic structure was investigated by X-ray absorption near-edge structure (XANES) analysis, proving the presence of unsaturated sulfur atoms and strong electronic coupling between MoS₂ and SWNT. This study provides an in-situ synthetic route to create new multifunctional flexible hybridized catalysts and useful insights into the relationships among the catalyst microstructure, electronic structure, and properties.

Open image in new window

     

MoS<Subscript>2</Subscript>/CoS<Subscript>2</Subscript> composites composed of CoS<Subscript>2</Subscript> octahedrons and MoS<Subscript>2</Subscript> nano-flowers for supercapacitor electrode materials

In pursuing excellent supercapacitor electrodes, we designed a series of MoS₂/CoS₂ composites consisting of flower-liked MoS₂ and octahedron-shaped CoS₂ through a facile one-step hydrothermal method and investigated the electrochemical performance of the samples with various hydrothermal time. Due to the coupling of two metal species and a big amount of well-developed CoS₂ and MoS₂, the results indicated that the MoS₂/CoS₂ composites electrodes exhibited the best electrochemical performance with a large specific capacitance of 490 F/g at 2 mV/s or 400 F/g at 10 A/g among all samples as the hydrothermal time reached 48 h (MCS₄₈). Furthermore, the retention of MCS₄₈ is 93.1% after 10000 cycles at 10 A/g, which manifests the excellent cycling stability. The outstanding electrochemical performance of MCS₄₈ indicates that it could be a very promising and novel energy storage material for supercapacitors in the future.     

Enhancement in electrical conductivity of Li<Subscript>2</Subscript>O: B<Subscript>2</Subscript>O<Subscript>3</Subscript>: V<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

The study of electrical conductivity of 30Li₂O: (70 − x) B₂O₃: xV₂O₅ glass samples has been carried out. The results have been explained by dividing the temperature range into two regions. In region I, conductivity shows Arrhenius behaviour for all the samples. The conductivity increases with addition of V₂O₅. The results have been explained in the light of Anderson and Stuart Model. In region II, an anomalous enhancement in the conductivity is observed for all the samples up to certain temperature beyond which the conductivity decreases. The enhancement in the conductivity in the annealed glass sample has been attributed to nanocrystallization.     

A novel method to control oxygen stoichiometry and thermoelectric properties in (RE)BaCo<Subscript>2</Subscript>O<Subscript>5+<Emphasis Type="Italic">δ</Emphasis></Subscript>

Rare earth cobaltites of the type (RE)BaCo₂O_5+δ (RE = Y, Gd, Eu and Nd) were synthesized by solid state technique. A novel, fast quenching technique was used to tune the oxygen content in these compounds. Room temperature Seebeck and electrical resistivity measurements were used to infer the oxygen content. A maximum in the S and ρ was observed for all the compositions when δ value was close to 0·5.     

Enhanced electrical and optical properties of single-layered MoS<Subscript>2</Subscript> by incorporation of aluminum

Electrical and optical enhancements of single-layer semiconducting materials such as transition metal dichalcogenides have recently been studied to achieve sensitive properties via external treatments, such as the formation of organic/inorganic protecting layers on field-effect transistors (FETs), thermal annealing, and nano-dot doping of sensors and detectors. Here, we propose a new analytical approach to electrical and optical enhancement through a passivation process using atomic layer deposition (ALD), and demonstrate a synthesized MoS₂ monolayer incorporated with Al atoms in an Al₂O₃ passivation layer. The incorporated Al atoms in the MoS₂ monolayer are clearly observed by spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM) and TEM-energy-dispersive X-ray spectroscopy results. We demonstrate that the chemically incorporated FETs exhibit highly enhanced mobilities of approximately 3.7 cm²·V^?1·s^?1, forty times greater than that of as-synthesized MoS₂, with a three-fold improvement in the photoluminescence properties.

Open image in new window

     

Responsivity to solar irradiation and the behavior of carrier transports for MoS<Subscript>2</Subscript>/Si and MoS<Subscript>2</Subscript>/Si nanowires/Si devices

The behavior of carrier transports and the responsivity to solar irradiation are studied for MoS₂/n-type Si (n-Si) and MoS₂/Si nanowires (SiNWs)/n-Si device. The MoS₂ thin films were prepared by the sol–gel method. The MoS₂/n-Si and MoS₂/SiNWs/n-Si devices exhibit reliable rectification behavior. The thermionic emission-diffusion model is the dominant process in these fabricated MoS₂/n-Si and MoS₂/SiNWs/n-Si devices. By applying the insertion of the SiNWs, the responsivity to solar irradiation can be effectively increased. Because of the low reflectance values for the MoS₂/SiNWs/n-Si sample, the increased photocurrent density for the MoS₂/SiNWs/n-Si device is due to high external light injection efficiency. MoS₂/SiNWs/n-Si devices benefit from the high surface to volume ratio for SiNWs leading to a high responsivity of the device. The photo-response results confirm that the decay in the photocurrent is due to the dominance of short-lifetime electron trapping in the MoS₂ thin film.     

MoS<Subscript>2</Subscript>-graphene in-plane contact for high interfacial thermal conduction

Recent studies have indicated that two-dimensional (2D) MoS₂ exhibits low in-plane and inter-plane thermal conductivities. This poses a significant challenge to heat management in MoS₂-based electronic devices. To address this challenge, we have designed MoS₂-graphene interfaces that fully utilize graphene, a 2D material that exhibits very high thermal conductivity. First, we performed ab initio atomistic simulations to understand bonding and structural stability at the interfaces. The interfaces that we designed, which were connected via strong covalent bonds between Mo and C atoms, were energetically stable. We then performed molecular dynamics simulations to investigate interfacial thermal conductance in these materials. Surprisingly, the interfacial thermal conductance was high and comparable to those of covalently bonded graphene-metal interfaces. Importantly, each interfacial Mo–C bond served as an independent thermal channel, enabling modulation of the interfacial thermal conductance by controlling the Mo vacancy concentration at the interface. The present work provides a viable heat management strategy for MoS₂-based electronic devices.

Open image in new window