Few-layer MoS2 Research Articles

S-sphere/C/MoS2 composite for high-performance Lithium–Sulfur batteries

With the development of energy storage systems, Lithium–Sulfur (Li–S) batteries represent an ideal choice due to extremely high theoretical specific capacity (1675 mAh/g−1) and energy density (2500 Wh/kg−1) of Sulfur cathode. However, it is a challenge to solve the electroconductibility of Sulfur, shuttle effect of polysulfide and volume expansion. Although tremendous works have been carried out to improve the capacity, the practical application of Li–S batteries is still hindered by low electroconductibility of Sulfur, poor cycle life and low rate performances. Herein, S-sphere/C/MoS2 composite was prepared by the melt diffusion method for the nano-sulfur host. In comparison with the large particles sublimate Sulfur, nano-sulfur particles can shorten the diffusion path of ions and facilitate the smooth diffusion of Li+ into the interior of Sulfur at high discharging current densities. Therefore, the formation of insulating Li2S2 and Li2S is delayed. Moreover, the Few-layered MoS2 can be used to achieve chemical adsorption with Lithium polysulfide (LiPS) and Carbon spheres has good electronic conductivity, thus the rate performance and the cyclic capability of Li–S batteries have been improved. The S-sphere/C/MoS2 composite exhibits an initial discharging capacity of 1016.3 mAh/g−1 at 0.1 C, and a high capacity of 753 mAh/g−1 at 1 C with a low fade rate of 0.211%. These indicate that the S-sphere/C/MoS2 composite has good electrochemical performance and is expected to be a candidate electrode for Li–S batteries industrialization in the future.

The impact of strain on growth mode in chemical vapor deposited mono- and few-layer MoS2

-The development of high-quality chemical vapor-deposited mono- and few-layer MoS2 is of high relevance for future applications in functional devices. Consequently, a detailed understanding of the growth mode and the parameters affecting it is important. Here, we show for the case of mono- and few-layer MoS2 grown on Muscovite mica, how strain and temperature impact the growth mode. We show how misleading the determination of the number of MoS2 layers is, solely based on Raman spectroscopy due to the occurrence of strain and changes in the growth mode. A combination of atomic force microscopy, Raman spectroscopy, and ab initio calculations reveal that that the growth at 500 °C synthesis temperature exhibits a strained layer-by-layer growth of up to three mono-layers, whereas at 700 °C, a strain release occurs and layer-by-layer growth is confined to the first mono-layer only. We relate the occurrence of strain to the formation of gas bubbles below the MoS2 film, escaping the mica sheets during high temperature synthesis. Our analysis shows that mica substrates can be used to study strain in 2D materials without the need to apply external stress and that a detailed knowledge of the MoS2 morphology is necessary to correctly interpret the Raman results.

Evaluation of TEM methods for their signature of the number of layers in mono- and few-layer TMDs as exemplified by MoS2 and MoTe2

In mono- and few-layer 2D materials, the exact number of layers is a critical parameter, determining the materials’ properties and thus their performance in future nano-devices. Here, we evaluate in a systematic manner the signature of exfoliated free-standing mono- and few-layer MoS2 and MoTe2 in TEM experiments such as high-resolution transmission electron microscopy, electron energy-loss spectroscopy, and 3D electron diffraction. A reference for the number of layers has been determined by optical contrast and AFM measurements on a substrate. Comparing the results, we discuss strengths and limitations, benchmarking the three TEM methods with respect to their ability to identify the exact number of layers.

Plasmonic enhancement of exciton and trion photoluminescence in 2D MoS2 decorated with Au nanorods: Impact of nonspherical shape

Two-dimensional (2D) MoS2 is emerging as a promising candidate for novel optoelectronics and flexible devices, but the photoluminescence (PL) efficiency is still limited for actual photonic applications. Plasmonic enhancement is the best way to increase the PL emission of 2D materials. In this study, the PL enhancement of 2D MoS2 decorated with Au nanorods (NRs) and nanospheres (NSs) at room temperature is investigated, focusing in particular on the effect of the nonspherical shape of NRs. To have a proper comparison, both nanoparticles have very similar diameters of 13–14 nm, while the NR average length is 81 nm. After decorating with Au NRs and NSs, the average PL enhancement of few-layer MoS2 is about 4.3 and 2 times, respectively. The Raman spectra are similarly enhanced, 3.8 and 2.4 times (A1g mode), respectively. The maximal enhancement is observed for the samples with Au NRs: this feature can be related to highly localized plasmonic hot spots in vicinity of the NR tips. A strong electric field within the hot spots causes the generation of additional free electron-hole pairs and, in turn, the enhancement of exciton excitation and the increase of the PL intensity. Simultaneously, the injection of hot electrons from the metal increases the probability of trions generation, thus contributing to the PL enhancement.

Spontaneous-polarization-induced photovoltaic effect in rhombohedrally stacked MoS2

Stacking order in van der Waals materials determines the coupling between atomic layers and is therefore key to the materials' properties. By exploring different stacking orders, many novel physical phenomena have been realized in artificial vdW stacks. Recently, 2D ferroelectricity has been observed in zero-degree aligned hBN and graphene-hBN heterostructures, holding promise in a range of electronic applications. In those artificial stacks, however, the single domain size is limited by the stacking-angle misalignment to about 0.1 to 1 $\mu$m, which is incompatible with most optical or optoelectronic applications. Here we show MoS$_2$ in the rhombohedral phase can host a homogeneous spontaneous polarization throughout few-$\mu$m-sized exfoliated flakes, as it is a natural crystal requiring no stacking and is, therefore free of misalignment. Utilizing this homogeneous polarization and its induced depolarization field (DEP), we build a graphene-MoS$_2$ based photovoltaic device with high efficiency. The few-layer MoS$_2$ is thinner than most oxide-based ferroelectric films, which allows us to maximize the DEP and study its impact at the atomically thin limit, while the highly uniform polarization achievable in the commensurate crystal enables a tangible path for up-scaling. The external quantum efficiency of our device is up to 16% at room temperature, over one order larger than the highest efficiency observed in bulk photovoltaic devices, owing to the reduced screening in graphene, the exciton-enhanced light-matter interaction, and the ultrafast interlayer relaxation in MoS$_2$. In view of the wide range of bandgap energy in other TMDs, our findings make rhombohedral TMDs a promising and versatile candidate for applications such as energy-efficient photo-detection with high speed and programmable polarity.

Anomalous enhancement oxidation of few-layer MoS2 and MoS2/h-BN heterostructure

Anomalous enhancement oxidation of few-layer MoS2 and MoS2/h-BN heterostructure

Compressible Neuron-like 3D Few-Layered MoS2/N-Doped Graphene Foam as Freestanding and Binder-Free Electrodes for High-Performance Lithium-Ion Batteries

Compressible Neuron-like 3D Few-Layered MoS₂/N-Doped Graphene Foam as Freestanding and Binder-Free Electrodes for High-Performance Lithium-Ion Batteries

DFT-based layered dielectric model of few-layer MoS2

We employ atomistic calculations to study charge distribution in few-layer MoS2 structures with an applied perpendicular electric field. The results suggest a simple continuum model consisting of alternating regions which represent the semiconductor layers and the Van der Waals gaps between them. Such model is a first step towards an accurate simulation of MoS2 in TCAD tools.

Boosting the photovoltaic performance of MoS2/Si heterojunction solar cells with thiourea-doped MoS2 films

Two-dimensional molybdenum disulfide film has unique physical and chemical properties, which make it a prominent candidate for optoelectronic device applications. In this work, the high-quality few-layer MoS2 films were successfully deposited by spin-coating the ammonium tetrathiomolybdate aqueous solution doped with an appropriate amount of thiourea on p-Si substrates. The effects of thiourea doping ratio on the microstructural properties of the prepared MoS2 films were analyzed by Raman spectrum, X-ray diffraction, and X-ray photoelectron spectroscopy. It was found that the few-layer MoS2 films with high crystallinity and low defect density can be formed at the thiourea doping ratio of 0.5. The electrical and photovoltaic properties of the MoS2/Si heterojunction solar cells fabricated by using the MoS2 film with different thiourea doping ratios were investigated by current density-voltage, external quantum efficiency, and capacitance-voltage measurements. The MoS2/Si heterojunction solar cell based on the optimized thiourea doping ratio achieves a power conversion efficiency of 9.81%. The results display a simple and convenient method for preparing the high-quality MoS2 films and boosting the performance of MoS2/Si heterojunction solar cells, providing a possibility to realize the low-cost and large-area MoS2-based optoelectronic devices.

Fabrication of three-dimensional nanocomposite with open cage fullerene polymer and MoS2 for enhanced electrochemical hydrogen evolution activity

Development of advanced materials for electrocatalytic water splitting to generate hydrogen is the key in utilization of renewable energy. In the present work, we constructed three-dimensional (3D) nanoparticles (P-PC60@MoS2) based on an open-cage fullerene polymer (P-PC60), on the surface of which two-dimensional (2D) molybdenum disulfide (MoS2) are grafted vertically. The rational design endows the nanocomposite materials with combined advantages of open-cage fullerenes and MoS2. Especially, compared with traditional fullerenes, polymeric fullerene derivatives are expected to have remarkable electronic, optical, or catalytic properties due to the formation of an interconnected continuous network. The results indicate that the few-layer MoS2 slices arranged vertically on the surface of the fullerene, showing an expanded layer spacing of 6.4 Å, which is beneficial for maximally exposing active edge sites. The hybrid P-PC60@MoS2 with proper synthetic engineering possesses superior catalytic kinetics with surpassing overpotential of 59 mV, Tafel slope of 37 mV dec−1, which are much lower than those of pure MoS2 and P-PC60. The small values indicate fast electrochemical hydrogen evolution reaction (HER) kinetics.

Direct Z-scheme MoSe2/TiO2 heterostructure with improved piezoelectric and piezo-photocatalytic performance

Nano-semiconductor materials coupled with piezoelectric effect have received extensive attention due to their wide application in catalysis. In this work, few-layered MoSe2 nanosheets were grown vertically on TiO2 nanorods (TNr) to synthesize a direct Z-scheme heterojunction, exhibiting efficient piezocatalytic and piezo-photocatalytic performance. The MoSe2/TNr heterostructure exhibited superior piezoelectric degradation efficiency, successfully removing over 98% of RhB within 360 s under continuous magnetic stirring in dark. Compared with piezocatalysis, the piezo-photocatalytic system possessed higher degradation efficiency and cycle stability. Furthermore, a piezo-photoelectric synergistic effect of nanocomposites was observed by current outputs. Under stirring conditions, the current density of depleted MoSe2/TNr and MoSe2 nanosheets were respectively 6.3 μA/cm2 and 5.5 μA/cm2. When light and stirring were applied, the MoSe2/TNr current density increased twice to 13.2 μA/cm2, while the MoSe2 nanosheets didn’t exhibit improvement. Through the direct Z-scheme heterojunction of MoSe2/TNr, photoexcitation and piezoelectric polarization work together to effectively replenish carriers under light irradiation, and then rapidly separate free charges through piezopotential. This work broadens the application prospects of piezocatalysis and piezo-photocatalysis in renewable energy harvesting and water purification.

Highly sensitive few-layer MoS2 nanosheets as a stable soil moisture and humidity sensor

In this work, an MoS2 nanosheet based ultrasensitive humidity and soil moisture sensor has been developed. MoS2 nanosheets are obtained by sonication based surfactant-induced exfoliation. The performance of the sensor is examined at room temperature (25 ∘C) with respect to various relative humidity (RH) levels and soil water content (θg). A 434 fold variation (35–15200 pF) in capacitance value is seen when the relative humidity level changes from 11% to 96%. The response and recovery times of the microsensor are approximately 30 s and 40 s, respectively. The capacitance of the MoS2 based sensor increases monotonically when the soil moisture content increases from 3.6% to 53% and 2.3–45% for black and red soil, respectively. The observed variation in capacitance corresponding to θgB = 53% (black soil moisture content) and θgR = 45% (red soil moisture content) is approximately 250 and 200 times that observed for the respective dry soil samples. The error in the soil moisture response measurement for the temperature variation from 25 ∘C to 70 ∘C is ± 2.52%. The device features stable performance for six months with reasonably high accuracy (maximum error ± 1.50%). The soil sensing results show fast response (35 s), negligible hysteresis error (2.80% at θgB = 31.9%), long-term stability (6 months and more), and good reproducibility (with more than 10 cycles).

Improved Temporal Response of MoS2 Photodetectors by Mild Oxygen Plasma Treatment.

Temporal response is an important factor limiting the performance of two-dimensional (2D) material photodetectors. The deep trap states caused by intrinsic defects are the main factor to prolong the response time. In this work, it is demonstrated that the trap states in 2D molybdenum disulfide (MoS2) can be efficiently modulated by defect engineering through mild oxygen plasma treatment. The response time of the few-layer MoS2 photodetector is accelerated by 2–3 orders of magnitude, which is mainly attributed to the deep trap states that can be easily filled when O2 or oxygen ions are chemically bonded with MoS2 at sulfur vacancies (SV) sites. We characterized the defect engineering of plasma-exposed MoS2 by Raman, PL and electric properties. Under the optimal processing conditions of 30 W, 50 Pa and 30 s, we found 30-fold enhancements in photoluminescence (PL) intensity and a nearly 2-fold enhancement in carrier field-effect mobility, while the rise and fall response times reached 110 ms and 55 ms, respectively, at the illumination wavelength of 532 nm. This work would, therefore, offer a practical route to improve the performance of 2D dichalcogenide-based devices for future consideration in optoelectronics research.

Size Effects in Single- and Few-Layer MoS2 Nanoflakes: Impact on Raman Phonons and Photoluminescence.

The high optical absorption and emission of bidimensional MoS2 are fundamental properties for optoelectronic and biodetection applications and the opportunity to retain these properties in high quality nano-sized flakes would bring further possibilities. Here, a large set of single-layer and few-layer (2–3 layers) MoS2 flakes with size in the range from 10 nm to 20 μm are obtained on sapphire by vapor deposition techniques and evaluated combining the information from the Raman phonons with photoluminescence (PL) and absorption bands. The flakes have triangular shape and are found to be progressively relaxed from the tensile strain imposed by the sapphire substrate as their size is reduced. An increasing hole doping as size decreases is deduced from the blue shift of the A1g phonon, related to charge transfer from adsorbed oxygen. No clear correlation is observed between defects density and size, therefore, doping would be favored by the preferential adsorption of oxygen at the edges of the flakes, being progressively more important as the edge/surface ratio is incremented. This hole doping also produces a shift of the PL band to higher energies, up to 60 meV. The PL intensity is not found to be correlated to the size but to the presence of defects. The trends with size for single-layer and for 2–3 layer samples are found to be similar and the synthesis method does not influence PL efficiency which remains high down to 40 nm being thus promising for nanoscale photonics.

Spontaneous Polarization induced Photovoltaic Effect In rhombohedrally stacked MoS2

Stacking order in van der Waals materials determines the coupling between atomic layers and is therefore key to the materials' properties. By exploring different stacking orders, many novel physical phenomena have been realized in artificial vdW stacks. Recently, 2D ferroelectricity has been observed in zero-degree aligned hBN and graphene-hBN heterostructures, holding promise in a range of electronic applications. In those artificial stacks, however, the single domain size is limited by the stacking-angle misalignment to about 0.1 to 1 $\mu$m, which is incompatible with most optical or optoelectronic applications. Here we show MoS$_2$ in the rhombohedral phase can host a homogeneous spontaneous polarization throughout few-$\mu$m-sized exfoliated flakes, as it is a natural crystal requiring no stacking and is, therefore free of misalignment. Utilizing this homogeneous polarization and its induced depolarization field (DEP), we build a graphene-MoS$_2$ based photovoltaic device with high efficiency. The few-layer MoS$_2$ is thinner than most oxide-based ferroelectric films, which allows us to maximize the DEP and study its impact at the atomically thin limit, while the highly uniform polarization achievable in the commensurate crystal enables a tangible path for up-scaling. The external quantum efficiency of our device is up to 16% at room temperature, over one order larger than the highest efficiency observed in bulk photovoltaic devices, owing to the reduced screening in graphene, the exciton-enhanced light-matter interaction, and the ultrafast interlayer relaxation in MoS$_2$. In view of the wide range of bandgap energy in other TMDs, our findings make rhombohedral TMDs a promising and versatile candidate for applications such as energy-efficient photo-detection with high speed and programmable polarity.

Few-Layered MoS2/ZnCdS/ZnS Heterostructures with an Enhanced Photocatalytic Hydrogen Evolution

Studies on highly efficient noble-metal-free catalysts are regarded as an important task for H2 production by water-splitting. MoS2/ZnCdS/ZnS dual heterostructures were successfully prepared with respective supernatant MoS2 colloidal solutions (called M/ZC/Z) and MoS2 precipitates (M(p)/ZC/Z). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the M/ZC/Z sample showed that ZnS nanosheets were decorated with ZnCdS nanorods and few-layered MoS2. Then, photocatalytic hydrogen production study was performed and the results showed that the M/ZC/Z sample has a H2 evolution rate of up to 79.3 mmol g–1 h–1 under visible light irradiation with an apparent quantum efficiency of 47.9% at 420 nm with noble-metal-free catalysts, which is nearly 5 times that of the M(p)/ZC/Z sample (15.7 mmol g–1 h–1) and approximately 9 times that of the ZC/Z sample (8.98 mmol g–1 h–1). Cycle tests showed that M/ZC/Z is stable and reusable. Without sacrificial agents, the production rates for hydrogen and oxygen evolution were obtained as 3.15 and 1.55 mmol g–1 h–1, respectively. Time-resolved photoluminescence spectra revealed that the well-matched structure is effective in the separation and transfer of photogenerated electron and hole pairs, leading to the enhancement of the photocatalytic H2 production activity.

Different strategies for GaN-MoS2 and GaN-WS2 core–shell nanowire growth

One-dimensional (1D) nanostructures – nanowires (NWs) – exhibit attractive properties for integration in different types of functional devices. Their properties can be enhanced even further or tuned for a specific application by combining different promising materials, such as layered van der Waals materials and conventional semiconductors, into 1D-1D core–shell heterostructures. In this work, we demonstrated growth of GaN-MoS2 and GaN-WS2 core–shell NWs via two different methods: (1) two-step process of sputter-deposition of a sacrificial transition metal oxide coating on GaN NWs followed by sulfurization; (2) pulsed laser deposition of few-layer MoS2 or WS2 on GaN NWs from the respective material targets. As-prepared nanostructures were characterized via scanning and transmission electron microscopies, X-ray diffraction, micro-Raman spectroscopy and X-ray photoelectron spectroscopy. High crystalline quality core–shell NW heterostructures with few-layer MoS2 and WS2 shells can be prepared via both routes. The experimental results were supported by theoretical electronic structure calculations, which demonstrated the potential of the synthesised core–shell NW heterostructures as photocatalysts for efficient hydrogen production from water.

Constructing highly stable lithium storage materials by improving the bond strength of MoS2 to graphene via chitosan

MoS2 is considered a promising substitute anode for lithium-ion batteries (LIBs) due to its two-dimensional layered structure and large theoretical specific capacity. However, the low conductivity and poor structural stability of MoS2 limit its practical applications. Here, a simple self-assembly strategy is adopted to successfully synthesize MoS2/C@G with a 3D porous hybrid nanosheet structure. By utilizing the chitosan can effectively bonding the layered MoS2 with N-doped graphene to construct a layered sandwich-like structure, which ensures the uniform and non-agglomeration distribution of the synthesized few-layer MoS2 nanosheets between graphene sheets. In addition, the strong bond between MoS2 and graphene makes these two components tightly bonded, which greatly improves the electronic conduction and guarantees structural stability. Based on the above advantages, MoS2/C@G as the electrode for LIBs exhibits high reversible capacity and superior cycling stability, with a capacity of 667.7 mAh g−1 at a high current density of 0.5 A g−1 after ultra-long 1000 cycles, even without significant capacity decay. This method also provides an advanced perspective for the fabrication of high-performance graphene composite anode materials.

Lattice origin of few-layer edge-on MoS2@TiO2 octahedral clusters for piezoelectric enhancement

Imaging the lattice origin of few-layer edge-on MoS2/TiO2 heterostructure at different growth stages could provide direct information for understanding the mechanism of its enhanced piezoelectric property. Via modifying the substrate surface to grow anatase TiO2 nanoparticles with truncated octahedron structure, few-layer MoS2 was exquisitely grown along the {101} basal planes of TiO2 by Van der Waals contact, and finally form an edge-on structure at the top edge of TiO2 nanoparticles due to steric hindrance. The vertical growth of few-layer MoS2 on the TiO2 surface endows not only better electronic contact between MoS2 and TiO2 substrate for fast electrons transfer but also high structure stability of piezoelectric properties. The piezocatalytic degradation ability is enhanced with the increase of hydrothermal reaction time, and the MoS2/TiO2-24 h sample exhibits the highest piezocatalytic activity under dark condition which demonstrates a high removal efficiency of 96.0% Rhodamine B within 60 min. This study establishes a relationship between the piezoelectric property of few-layer edge-on MoS2/TiO2 heterostructure and its growth mechanisms, and sheds light on the rational design of electric contact between transition metal dichalcogenide and semiconductor with specific crystal plane which is significant to the improvement of carrier separation and transport.

Fully Integrated Photodetector Array Based on an Electrochemically Exfoliated, Atomically Thin MoS2 Film for Photoimaging

Despite the widespread interest in MoS2-based photodetectors over the past few years, use of a high-performance photoimaging device remains a major challenge due to the low yield and poor quality of the MoS2 film. In this paper, we demonstrate a simple method to fabricate a high-performance fully integrated photodetector array based on a large-area atomically thin MoS2 film for photoimaging. Here, the film is constructed by spin-coating of the MoS2 nanosheets, which are acquired via electrochemical intercalation and sonication-assisted exfoliation. By precisely controlling this process, semiconducting few-layered MoS2 nanosheets with pure phase can be obtained. High-performance photodetectors have been achieved for the TFSI-treated MoS2 film, as the responsivity reaches 1080 A/W, and the response/recovery time improved to 56/62 ms with high stability during the repeated on/off test. The overall performance is much better than that of previously reported photodetectors based on liquid-exfoliated MoS2 nanosheets. Furthermore, a fully integrated photoimaging device with a crossing structure and an effective pixel density of 460/cm2 has been fabricated based on the MoS2 film, and shows great potential for shape imaging with cross-talk suppression. We believe this work provides an approach to exploit the photodetector and image sensor based on layered materials.