MoS2

/Tag: MoS2

Ultrafast photocarrier recombination dynamics in black phosphorus-molybdenum disulfide (BP/MoS2) heterostructure. (arXiv:1811.04706v1 [cond-mat.mes-hall])

2018-11-13T04:30:17+00:00November 13th, 2018|Categories: Publications|Tags: , , , |

Van der Waals (vdW) heterostructures constructed with two-dimensional (2D) materials have attracted great interests, due to their fascinating properties and potential for novel applications. While earlier efforts have advanced the understanding of the ultrafast cross-layer charge transfer process in 2D heterostructures, mechanisms for the interfacial photocarrier recombination remain, to a large extent, unclear. Here, we investigate a heterostructure comprised of black phosphorus (BP) and molybdenum disulfide (MoS2), with a type-II band alignment. Interestingly, it is found that the photo-generated electrons in MoS2 (transferred from BP) exhibit an ultrafast lifetime of about 5 ps, significantly shorter than those of the constituent materials. By corroborating with the relaxation of photo-excited holes in BP, it is revealed that the ultrafast time constant is as a result of efficient Langevin recombination, where the high hole mobility of BP facilitates a large recombination coefficient (approximately 2×10^-10 m^2/s). In addition, broadband transient absorption spectroscopy confirms that the hot electrons transferred to MoS2 distribute over a broad energy range following an ultrafast thermalization. The rate of the interlayer Langevin recombination is found to exhibit no energy level dependence. Our findings provide essential insights into the fundamental photo-physics in type-II 2D heterostructures, and also provide useful guidelines for customizing photocarrier lifetimes of BP for high-speed photo-sensitive devices.

Published : "arXiv Mesoscale and Nanoscale Physics".

Atomic process of oxidative etching in monolayer molybdenum disulfide. (arXiv:1811.04242v1 [cond-mat.mtrl-sci])

2018-11-13T02:29:19+00:00November 13th, 2018|Categories: Publications|Tags: , , , |

The microscopic process of oxidative etching of two-dimensional molybdenum disulfide (2D MoS2) at an atomic scale is investigated using a correlative TEM-etching study. MoS2 flakes on graphene TEM grids are precisely tracked and characterized by TEM before and after the oxidative etching. This allows us to determine the structural change with an atomic resolution on the edges of the domains, of well-oriented triangular pits and along the grain boundaries. We observe that the etching mostly starts from the open edges, grain boundaries and pre-existing atomic defects. A zigzag Mo edge is assigned as the dominant termination of the triangular pits, and profound terraces and grooves are observed on the etched edges. Based on the statistical TEM analysis, we reveal possible routes for the kinetics of the oxidative etching in 2D MoS2, which should also be applicable for other 2D transition metal dichalcogenide materials like MoSe2 and WS2.

Published in: "arXiv Material Science".

Functionalized MoS2 Nanovehicle with Near‐Infrared Laser‐Mediated Nitric Oxide Release and Photothermal Activities for Advanced Bacteria‐Infected Wound Therapy

2018-11-11T14:34:25+00:00November 11th, 2018|Categories: Publications|Tags: |

A novel 808 nm laser‐mediated nitric oxide (NO)‐releasing nanovehicle (MoS2‐BNN6) based on simple assembly of α‐ cyclodextrin‐modified MoS2 nanosheets with a heat‐sensitive NO donor (BNN6) is developed. The nanovehicle possesses a synergetic photothermal therapy/NO antibacterial function for low‐cost, rapid, and effective action against typical Gram’s bacteria as well as promotion of wound healing. Abstract The rising dangers of bacterial infections have created an urgent need for the development of a new generation of antibacterial nanoagents and therapeutics. A new near‐infrared 808 nm laser‐mediated nitric oxide (NO)‐releasing nanovehicle (MoS2‐BNN6) is reported through the simple assembly of α‐cyclodextrin‐modified MoS2 nanosheets with a heat‐sensitive NO donor N,N′‐di‐sec‐butyl‐N,N′‐dinitroso‐1,4‐phenylenediamine (BNN6) for the rapid and effective treatment of three typical Gram‐negative and Gram‐positive bacteria (ampicillin‐resistant Escherichia coli, heat‐resistant Escherichia faecalis, and pathogen Staphylococcus aureus). This MoS2‐BNN6 nanovehicle has good biocompatibility and can be captured by bacteria to increase opportunities of NO diffusion to the bacterial surface. Once stimulated by 808 nm laser irradiation, the MoS2‐BNN6 nanovehicle not only exhibits photothermal therapy (PTT) efficacy but also can precisely control NO release, generating oxidative/nitrosative stress. The temperature‐enhanced catalytic function of MoS2 induced by 808 nm laser irradiation simultaneously accelerates the oxidation of glutathione. This acceleration disrupts the balance of antioxidants, ultimately resulting in significant DNA damage to the bacteria. Within 10 min, the MoS2‐BNN6 with enhanced PTT/NO synergetic antibacterial function achieves >97.2% inactivation of bacteria. The safe synergetic therapy strategy can also effectively repair wounds through the formation of collagen fibers and elimination of inflammation during tissue

Published in: "Small".

Charge Transfer within the F4TCNQ‐MoS2 van der Waals Interface: Toward Electrical Properties Tuning and Gas Sensing Application

2018-11-11T04:32:25+00:00November 11th, 2018|Categories: Publications|Tags: , |

Employing the organic charge transfer material 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane, the electrical properties of MoS2 field‐effect transistors including V on’s and subthreshold swings are successfully modulated with no degradation of mobility. The charge transfer mechanism is investigated by first‐principles calculation and scanning Kelvin probe microscopy. A high‐performance NH3 gas sensor fabricated from this transistor reaches more than 1000% sensitivity at 100 ppm. Abstract The development of van der Waals heterostructures in 2D materials systems has attracted considerable interests for exploring new insights of (opto‐) electrical characteristics, device physics, and novel functional applications. Utilizing organic molecular material with strong electron withdrawing ability, charge transfer van der Waals interfaces are formed between 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) and MoS2, via which the modulation of the onset voltages and optimization of subthreshold swing values in MoS2‐based field effect transistors are realized. Charge transfer process and its functionality mechanisms are further verified and investigated with first‐principles calculation, scanning Kelvin probe microscope characterization, and temperature‐dependent electrical characterization. With the charge transfer effect between reducing gas molecules and F4TCNQ, NH3 gas sensor is proposed and fabricated with the sensitivity reaching higher than 1000% at 100 ppm, much more outstanding performance than those of any reported MoS2‐based NH3 gas sensors. The F4TCNQ‐MoS2 hybrid strategy might open up a pathway for tuning and optimizing the electrical properties, in addition to novel functional units designing and fabrications in electric devices based on low‐dimensional semiconducting systems.

Published in: "Advanced Functional Materials".

Surface Modulation of Hierarchical MoS2 Nanosheets by Ni Single Atoms for Enhanced Electrocatalytic Hydrogen Evolution

2018-11-11T04:32:16+00:00November 11th, 2018|Categories: Publications|Tags: , |

Surface modulation at the atomic level has been an important approach for boosting the performance of electrocatalysts. Here, a combined theoretical and experimental study on Ni atom decorated hierarchical MoS2 nanosheets supported on a multichannel carbon matrix (MCM) is presented. The obtained hybrid [email protected]–Ni electrocatalyst with activated S sites exhibits high performance in electrocatalytic hydrogen evolution. Abstract Surface modulation at the atomic level is an important approach for tuning surface chemistry and boosting the catalytic performance. Here, a surface modulation strategy is demonstrated through the decoration of isolated Ni atoms onto the basal plane of hierarchical MoS2 nanosheets supported on multichannel carbon nanofibers for boosted hydrogen evolution activity. X‐ray absorption fine structure investigation and density functional theory (DFT) calculation reveal that the MoS2 surface decorated with isolated Ni atoms displays highly strengthened H binding. Benefiting from the unique tubular structure and basal plane modulation, the newly developed MoS2 catalyst exhibits excellent hydrogen evolution activity and stability. This single‐atom modification strategy opens up new avenues for tuning the intrinsic catalytic activity toward electrocatalytic water splitting and other energy‐related processes.

Published in: "Advanced Functional Materials".

Lowering the Schottky Barrier Height by Graphene/Ag Electrodes for High‐Mobility MoS2 Field‐Effect Transistors

2018-11-10T22:35:03+00:00November 10th, 2018|Categories: Publications|Tags: , |

Contact resistance between the channel and electrodes in MoS2 devices is significantly reduced using a low work function metal (Ag) and graphene as an interfacial layer between MoS2 and Ag because the Schottky barrier height is lowered at the contacts. Using graphene/Ag contacts instead of Ti/Au improves the field‐effect mobility, on/off current ratio, and photoresponsivity of the devices. Abstract 2D transition metal dichalcogenides (TMDCs) have emerged as promising candidates for post‐silicon nanoelectronics owing to their unique and outstanding semiconducting properties. However, contact engineering for these materials to create high‐performance devices while adapting for large‐area fabrication is still in its nascent stages. In this study, graphene/Ag contacts are introduced into MoS2 devices, for which a graphene film synthesized by chemical vapor deposition (CVD) is inserted between a CVD‐grown MoS2 film and a Ag electrode as an interfacial layer. The MoS2 field‐effect transistors with graphene/Ag contacts show improved electrical and photoelectrical properties, achieving a field‐effect mobility of 35 cm2 V−1 s−1, an on/off current ratio of 4 × 108, and a photoresponsivity of 2160 A W−1, compared to those of devices with conventional Ti/Au contacts. These improvements are attributed to the low work function of Ag and the tunability of graphene Fermi level; the n‐doping of Ag in graphene decreases its Fermi level, thereby reducing the Schottky barrier height and contact resistance between the MoS2 and electrodes. This demonstration of contact interface engineering with CVD‐grown MoS2 and graphene is a key step toward the practical application of atomically thin TMDC‐based devices

Published in: "Advanced Materials".

Direct Chirality Recognition of Single‐Crystalline and Single‐Walled Transition Metal Oxide Nanotubes on Carbon Nanotube Templates

2018-11-10T22:34:51+00:00November 10th, 2018|Categories: Publications|Tags: , |

Single‐crystalline molybdenum trioxide nanotubes (MONTs) are fabricated on carbon nanotube (CNT) templates. Single‐crystalline and single‐walled MONTs are prepared by van der Waals epitaxy on CNT templates. The chiral angles of the MONTs can be directly observed under an electron microscope. The MONTs show a preferred orientation with the [001] direction along the CNT axis due to the anisotropic bending rigidity of the monolayers. Abstract Chirality is a significant structural feature for chemistry, biology, physics, and materials science, and especially determines the electrical, mechanical, and optical properties of diverse tubular structures, such as carbon nanotubes (CNTs). To recognize the chirality of nanotubes, templates are introduced as potential tools to obtain crystalline samples with visible chiral fringes under electron microscopes. However, few efforts show optimistic results, and new understanding is desired to control the sample quality with CNT templates. Here, a synthesis strategy of single‐crystalline molybdenum trioxide (α‐MoO3) nanotubes (MONTs) on CNT surfaces is reported to build a 1D van der Waals (vdW) heterostructure. The chirality of the MONTs can be directly “seen” and their structural selectivity is revealed. First, the centralized distribution of the chiral angles of the MONTs indicates a preferential orientation due to the anisotropic bending rigidity of the 2D layers. Then, the interlayer mismatching rejects the radial stacking of α‐MoO3 to maintain the single‐walled nature. These results provide a spontaneous strategy for the efficient recognition and control of chirality, and open up a new avenue for CNT‐based functional 1D vdW heterostructures.

Published in: "Advanced Materials".

Lowering the Schottky Barrier Height by Graphene/Ag Electrodes for High‐Mobility MoS2 Field‐Effect Transistors

2018-11-10T00:37:27+00:00November 9th, 2018|Categories: Publications|Tags: , |

Contact resistance between the channel and electrodes in MoS2 devices is significantly reduced using a low work function metal (Ag) and graphene as an interfacial layer between MoS2 and Ag because the Schottky barrier height is lowered at the contacts. Using graphene/Ag contacts instead of Ti/Au improves the field‐effect mobility, on/off current ratio, and photoresponsivity of the devices. Abstract 2D transition metal dichalcogenides (TMDCs) have emerged as promising candidates for post‐silicon nanoelectronics owing to their unique and outstanding semiconducting properties. However, contact engineering for these materials to create high‐performance devices while adapting for large‐area fabrication is still in its nascent stages. In this study, graphene/Ag contacts are introduced into MoS2 devices, for which a graphene film synthesized by chemical vapor deposition (CVD) is inserted between a CVD‐grown MoS2 film and a Ag electrode as an interfacial layer. The MoS2 field‐effect transistors with graphene/Ag contacts show improved electrical and photoelectrical properties, achieving a field‐effect mobility of 35 cm2 V−1 s−1, an on/off current ratio of 4 × 108, and a photoresponsivity of 2160 A W−1, compared to those of devices with conventional Ti/Au contacts. These improvements are attributed to the low work function of Ag and the tunability of graphene Fermi level; the n‐doping of Ag in graphene decreases its Fermi level, thereby reducing the Schottky barrier height and contact resistance between the MoS2 and electrodes. This demonstration of contact interface engineering with CVD‐grown MoS2 and graphene is a key step toward the practical application of atomically thin TMDC‐based devices

Published in: "Advanced Materials".

Stacking and interlayer electron transport in MoS2. (arXiv:1811.03483v1 [cond-mat.mtrl-sci])

2018-11-09T02:29:32+00:00November 9th, 2018|Categories: Publications|Tags: |

In this work, we investigate the effect of the stacking sequence in MoS2 multilayer systems on their electron transport properties, through first-principles simulations of structural and electron transport properties. We show that interlayer electron transport is highly sensitive to the stacking sequence of the multilayers, with specific sequences producing much higher electron transmission due to larger orbital interactions and band structure effects. These results explain contrasting experimental evidence on interlayer transport measurements as due to imperfect structural control, provide insight on modeling and suggest ways to improve the performance of electron devices based on MoS2 multilayer systems via multilayer structure engineering.

Published in: "arXiv Material Science".

Mapping multi-valley Lifshitz transitions induced by field-effect doping in strained MoS2 nanolayers. (arXiv:1811.02978v1 [cond-mat.mes-hall])

2018-11-08T02:29:27+00:00November 8th, 2018|Categories: Publications|Tags: , |

Gate-induced superconductivity at the surface of nanolayers of semiconducting transition metal dichalcogenides (TMDs) has attracted a lot of attention in recent years, thanks to the sizeable transition temperature, robustness against in-plane magnetic fields beyond the Pauli limit, and hints to a non-conventional nature of the pairing. A key information necessary to unveil its microscopic origin is the geometry of the Fermi surface hosting the Cooper pairs as a function of field-effect doping, which is dictated by the filling of the inequivalent valleys at the K/K$^{prime}$ and Q/Q$^{prime}$ points of the Brillouin Zone. Here, we achieve this by combining Density Functional Theory calculations of the bandstructure with transport measurements on ion-gated 2H-MoS$_{2}$ nanolayers. We show that, when the number of layers and the amount of strain are set to their experimental values, the Fermi level crosses the bottom of the high-energy valleys at Q/Q$^{prime}$ at doping levels where characteristic kinks in the transconductance are experimentally detected. We also develop a simple 2D model which is able to quantitatively describe the broadening of the kinks observed upon increasing temperature. We demonstrate that this combined approach can be employed to map the dependence of the Fermi surface of TMD nanolayers on field-effect doping, detect Lifshitz transitions, and provide a method to determine the amount of strain and spin-orbit splitting between sub-bands from electric transport measurements in real devices.

Published in: "arXiv Material Science".

Unsaturated Sulfur Edge Engineering of Strongly Coupled MoS2 Nanosheet–Carbon Macroporous Hybrid Catalyst for Enhanced Hydrogen Generation

2018-11-07T14:33:13+00:00November 7th, 2018|Categories: Publications|Tags: , |

A class of strongly coupled MoS2 nanosheet–carbon macroporous hybrids with engineered unsaturated sulfur edges has been developed as excellent hydrogen evolution reaction (HER) electrocatalysts. Both experimental analysis and first‐principles calculations verify that the resultant catalysts exhibit high exposure of unsaturated sulfur edges and an optimized hydrogen adsorption free energy, greatly boosting the HER activity and durability in acidic and alkaline media. Abstract The low hydrogen adsorption free energy and strong acid/alkaline resistance of layered MoS2 render it an excellent pH‐universal electrocatalyst for hydrogen evolution reaction (HER). However, the catalytic activity is dominantly suppressed by its limited active‐edge‐site density. Herein, a new strategy is reported for making a class of strongly coupled MoS2 nanosheet–carbon macroporous hybrid catalysts with engineered unsaturated sulfur edges for boosting HER catalysis by controlling the precursor decomposition and subsequent sodiation/desodiation. Both surface chemical state analysis and first‐principles calculations verify that the resultant catalysts exhibit a desirable valence‐electron state with high exposure of unsaturated sulfur edges and an optimized hydrogen adsorption free energy, significantly improving the intrinsic HER catalytic activity. Such an electrocatalyst exhibits superior and stable catalytic activity toward HER with small overpotentials of 136 mV in 0.5 m H2SO4 and 155 mV in 1 m KOH at 10 mA cm−2, which is the best report for MoS2–C hybrid electrocatalysts to date. This work paves a new avenue to improve the intrinsic catalytic activity of 2D materials for hydrogen generation.

Published in: "Advanced Energy Materials".

Direct Chirality Recognition of Single‐Crystalline and Single‐Walled Transition Metal Oxide Nanotubes on Carbon Nanotube Templates

2018-11-07T10:34:06+00:00November 7th, 2018|Categories: Publications|Tags: , |

Single‐crystalline molybdenum trioxide nanotubes (MONTs) are fabricated on carbon nanotube (CNT) templates. Single‐crystalline and single‐walled MONTs are prepared by van der Waals epitaxy on CNT templates. The chiral angles of the MONTs can be directly observed under an electron microscope. The MONTs show a preferred orientation with the [001] direction along the CNT axis due to the anisotropic bending rigidity of the monolayers. Abstract Chirality is a significant structural feature for chemistry, biology, physics, and materials science, and especially determines the electrical, mechanical, and optical properties of diverse tubular structures, such as carbon nanotubes (CNTs). To recognize the chirality of nanotubes, templates are introduced as potential tools to obtain crystalline samples with visible chiral fringes under electron microscopes. However, few efforts show optimistic results, and new understanding is desired to control the sample quality with CNT templates. Here, a synthesis strategy of single‐crystalline molybdenum trioxide (α‐MoO3) nanotubes (MONTs) on CNT surfaces is reported to build a 1D van der Waals (vdW) heterostructure. The chirality of the MONTs can be directly “seen” and their structural selectivity is revealed. First, the centralized distribution of the chiral angles of the MONTs indicates a preferential orientation due to the anisotropic bending rigidity of the 2D layers. Then, the interlayer mismatching rejects the radial stacking of α‐MoO3 to maintain the single‐walled nature. These results provide a spontaneous strategy for the efficient recognition and control of chirality, and open up a new avenue for CNT‐based functional 1D vdW heterostructures.

Published in: "Advanced Materials".

Rhenium‐Doped and Stabilized MoS2 Atomic Layers with Basal‐Plane Catalytic Activity

2018-11-07T08:35:46+00:00November 7th, 2018|Categories: Publications|Tags: , |

Re‐doped MoS2 atomic layers in the distorted tetragonal structure show excellent activity and stability for electrocatalytic hydrogen production. Atomic‐level scanning transmission electron microscopy combined with density functional theory calculations reveal active local Mo‐rich structures and explain the best performance in Re0.55Mo0.45S2. The study provides a new catalyst design strategy through chemical doping. Abstract The development of stable and efficient hydrogen evolution reaction (HER) catalysts is essential for the production of hydrogen as a clean energy resource. A combination of experiment and theory demonstrates that the normally inert basal planes of 2D layers of MoS2 can be made highly catalytically active for the HER when alloyed with rhenium (Re). The presence of Re at the ≈50% level converts the material to a stable distorted tetragonal (DT) structure that shows enhanced HER activity as compared to most of the MoS2‐based catalysts reported in the literature. More importantly, this new alloy catalyst shows much better stability over time and cycling than lithiated 1T‐MoS2. Density functional theory calculations find that the role of Re is only to stabilize the DT structure, while catalysis occurs primarily in local Mo‐rich DT configurations, where the HER catalytic activity is very close to that in Pt. The study provides a new strategy to improve the overall HER performance of MoS2‐based materials via chemical doping.

Published in: "Advanced Materials".

High‐Performance Wafer‐Scale MoS2 Transistors toward Practical Application

2018-11-07T08:35:08+00:00November 7th, 2018|Categories: Publications|Tags: |

An improved synthesis strategy for wafer‐scale MoS2 continuous films by introducing multilayer MoS2 islands to improve the electrical contact and dielectric deposition is developed. The multilayer islands provide an extra amount of exposed edges to guarantee conformal contact with evaporated metal electrodes, while the bottom continuous 1L‐MoS2 serves as a high‐quality field effect transistor channel. Abstract Atomic thin transition‐metal dichalcogenides (TMDs) are considered as an emerging platform to build next‐generation semiconductor devices. However, to date most devices are still based on exfoliated TMD sheets on a micrometer scale. Here, a novel chemical vapor deposition synthesis strategy by introducing multilayer (ML) MoS2 islands to improve device performance is proposed. A four‐probe method is applied to confirm that the contact resistance decreases by one order of magnitude, which can be attributed to a conformal contact by the extra amount of exposed edges from the ML‐MoS2 islands. Based on such continuous MoS2 films synthesized on a 2 in. insulating substrate, a top‐gated field effect transistor (FET) array is fabricated to explore key metrics such as threshold voltage (V T) and field effect mobility (μFE) for hundreds of MoS2 FETs. The statistical results exhibit a surprisingly low variability of these parameters. An average effective μFE of 70 cm2 V−1 s−1 and subthreshold swing of about 150 mV dec−1 are extracted from these MoS2 FETs, which are comparable to the best top‐gated MoS2 FETs achieved by mechanical exfoliation. The result is a key step toward scaling 2D‐TMDs into functional systems and paves the way

Published in: "Small".

A Pseudolayered MoS2 as Li‐Ion Intercalation Host with Enhanced Rate Capability and Durability

2018-11-07T08:34:58+00:00November 7th, 2018|Categories: Publications|Tags: , |

The MoO x(OH) y pillars in O‐MoS2 interlayers lead to a pseudolayered structure with an expanded layer spacing of 10.15 Å, much larger than the value of pristine MoS2 (6.27 Å). The larger interlayer spacing leads to better Li‐ion intercalation kinetics. Meantime, the pillars can also tense the expanded MoS2 layers to avoid exfoliation during Li‐ion intercalation/deintercalation. Abstract As a popular strategy, interlayer expansion significantly improves the Li‐ion diffusion kinetics in the MoS2 host, while the large interlayer spacing weakens the van der Waals force between MoS2 monolayers, thus harming its structural stability. Here, an oxygen‐incorporated MoS2 (O‐MoS2)/graphene composite as a self‐supported intercalation host of Li‐ion is prepared. The composite delivers a specific capacity of 80 mAh g−1 in only 36 s at a mass loading of 1 mg cm−2, and it can be cycled 3000 times (over 91% capacity retention) with a 5 mg cm−2 loading at 2 A g−1. The O‐MoS2 exhibits a dominant 1T phase with an expanded layer spacing of 10.15 Å, leading to better Li‐ion intercalation kinetics compared with pristine MoS2. Furthermore, ex situ X‐ray diffraction tests indicate that O‐MoS2 sustains a stable structure in cycling compared with the gradual collapse of pristine MoS2, which suffers from excessive lattice breathing. Density functional theory calculations suggest that the MoO x(OH) y pillars in O‐MoS2 interlayers not only expand the layer spacing, but also tense the MoS2 layers to avoid exfoliation in cycling. Therefore, the O‐MoS2 shows a pseudolayered structure, leading to remarkable durability besides the

Published in: "Small".

Direct Chirality Recognition of Single‐Crystalline and Single‐Walled Transition Metal Oxide Nanotubes on Carbon Nanotube Templates

2018-11-07T00:35:26+00:00November 6th, 2018|Categories: Publications|Tags: , |

Single‐crystalline molybdenum trioxide nanotubes (MONTs) are fabricated on carbon nanotube (CNT) templates. Single‐crystalline and single‐walled MONTs are prepared by van der Waals epitaxy on CNT templates. The chiral angles of the MONTs can be directly observed under an electron microscope. The MONTs show a preferred orientation with the [001] direction along the CNT axis due to the anisotropic bending rigidity of the monolayers. Abstract Chirality is a significant structural feature for chemistry, biology, physics, and materials science, and especially determines the electrical, mechanical, and optical properties of diverse tubular structures, such as carbon nanotubes (CNTs). To recognize the chirality of nanotubes, templates are introduced as potential tools to obtain crystalline samples with visible chiral fringes under electron microscopes. However, few efforts show optimistic results, and new understanding is desired to control the sample quality with CNT templates. Here, a synthesis strategy of single‐crystalline molybdenum trioxide (α‐MoO3) nanotubes (MONTs) on CNT surfaces is reported to build a 1D van der Waals (vdW) heterostructure. The chirality of the MONTs can be directly “seen” and their structural selectivity is revealed. First, the centralized distribution of the chiral angles of the MONTs indicates a preferential orientation due to the anisotropic bending rigidity of the 2D layers. Then, the interlayer mismatching rejects the radial stacking of α‐MoO3 to maintain the single‐walled nature. These results provide a spontaneous strategy for the efficient recognition and control of chirality, and open up a new avenue for CNT‐based functional 1D vdW heterostructures.

Published in: "Advanced Materials".

Unsaturated Sulfur Edge Engineering of Strongly Coupled MoS2 Nanosheet–Carbon Macroporous Hybrid Catalyst for Enhanced Hydrogen Generation

2018-11-07T00:33:09+00:00November 6th, 2018|Categories: Publications|Tags: , |

A class of strongly coupled MoS2 nanosheet–carbon macroporous hybrids with engineered unsaturated sulfur edges has been developed as excellent hydrogen evolution reaction (HER) electrocatalysts. Both experimental analysis and first‐principles calculations verify that the resultant catalysts exhibit high exposure of unsaturated sulfur edges and an optimized hydrogen adsorption free energy, greatly boosting the HER activity and durability in acidic and alkaline media. Abstract The low hydrogen adsorption free energy and strong acid/alkaline resistance of layered MoS2 render it an excellent pH‐universal electrocatalyst for hydrogen evolution reaction (HER). However, the catalytic activity is dominantly suppressed by its limited active‐edge‐site density. Herein, a new strategy is reported for making a class of strongly coupled MoS2 nanosheet–carbon macroporous hybrid catalysts with engineered unsaturated sulfur edges for boosting HER catalysis by controlling the precursor decomposition and subsequent sodiation/desodiation. Both surface chemical state analysis and first‐principles calculations verify that the resultant catalysts exhibit a desirable valence‐electron state with high exposure of unsaturated sulfur edges and an optimized hydrogen adsorption free energy, significantly improving the intrinsic HER catalytic activity. Such an electrocatalyst exhibits superior and stable catalytic activity toward HER with small overpotentials of 136 mV in 0.5 m H2SO4 and 155 mV in 1 m KOH at 10 mA cm−2, which is the best report for MoS2–C hybrid electrocatalysts to date. This work paves a new avenue to improve the intrinsic catalytic activity of 2D materials for hydrogen generation.

Published in: "Advanced Energy Materials".

Charge Transfer within the F4TCNQ‐MoS2 van der Waals Interface: Toward Electrical Properties Tuning and Gas Sensing Application

2018-11-07T00:32:44+00:00November 6th, 2018|Categories: Publications|Tags: , |

Employing the organic charge transfer material 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane, the electrical properties of MoS2 field‐effect transistors including V on’s and subthreshold swings are successfully modulated with no degradation of mobility. The charge transfer mechanism is investigated by first‐principles calculation and scanning Kelvin probe microscopy. A high‐performance NH3 gas sensor fabricated from this transistor reaches more than 1000% sensitivity at 100 ppm. Abstract The development of van der Waals heterostructures in 2D materials systems has attracted considerable interests for exploring new insights of (opto‐) electrical characteristics, device physics, and novel functional applications. Utilizing organic molecular material with strong electron withdrawing ability, charge transfer van der Waals interfaces are formed between 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) and MoS2, via which the modulation of the onset voltages and optimization of subthreshold swing values in MoS2‐based field effect transistors are realized. Charge transfer process and its functionality mechanisms are further verified and investigated with first‐principles calculation, scanning Kelvin probe microscope characterization, and temperature‐dependent electrical characterization. With the charge transfer effect between reducing gas molecules and F4TCNQ, NH3 gas sensor is proposed and fabricated with the sensitivity reaching higher than 1000% at 100 ppm, much more outstanding performance than those of any reported MoS2‐based NH3 gas sensors. The F4TCNQ‐MoS2 hybrid strategy might open up a pathway for tuning and optimizing the electrical properties, in addition to novel functional units designing and fabrications in electric devices based on low‐dimensional semiconducting systems.

Published in: "Advanced Functional Materials".

Surface Modulation of Hierarchical MoS2 Nanosheets by Ni Single Atoms for Enhanced Electrocatalytic Hydrogen Evolution

2018-11-07T00:32:20+00:00November 6th, 2018|Categories: Publications|Tags: , |

Surface modulation at the atomic level has been an important approach for boosting the performance of electrocatalysts. Here, a combined theoretical and experimental study on Ni atom decorated hierarchical MoS2 nanosheets supported on a multichannel carbon matrix (MCM) is presented. The obtained hybrid [email protected]–Ni electrocatalyst with activated S sites exhibits high performance in electrocatalytic hydrogen evolution. Abstract Surface modulation at the atomic level is an important approach for tuning surface chemistry and boosting the catalytic performance. Here, a surface modulation strategy is demonstrated through the decoration of isolated Ni atoms onto the basal plane of hierarchical MoS2 nanosheets supported on multichannel carbon nanofibers for boosted hydrogen evolution activity. X‐ray absorption fine structure investigation and density functional theory (DFT) calculation reveal that the MoS2 surface decorated with isolated Ni atoms displays highly strengthened H binding. Benefiting from the unique tubular structure and basal plane modulation, the newly developed MoS2 catalyst exhibits excellent hydrogen evolution activity and stability. This single‐atom modification strategy opens up new avenues for tuning the intrinsic catalytic activity toward electrocatalytic water splitting and other energy‐related processes.

Published in: "Advanced Functional Materials".

Excitonic emission in van-der-Waals nanotubes of transition metal dichalcogenides. (arXiv:1811.01195v1 [cond-mat.mtrl-sci])

2018-11-06T05:29:32+00:00November 6th, 2018|Categories: Publications|Tags: , |

Nanotubes (NTs) of transition metal dichalcogenides (TMDs), such as MoS2 and WS2, were first synthesized more than a quarter of a century ago; nevertheless, many of their properties have so far remained basically unknown. This review presents the state of the art in the knowledge of the optical properties of TMD NTs. We first evaluate general properties of multilayered TMD crystals, and analyze available data on electronic band structure and optical properties of related NTs. Then, the technology for the formation and the structural characteristics of TMD NTs are represented, focusing on the structures synthesized by chemical transport reaction. The core of this work is the presentation of the ability of TMD NTs to emit bright photoluminescence (PL), which has been discovered recently. By means of micro-PL spectroscopy of individual tubes we show that excitonic transitions relevant to both direct and indirect band gaps contribute to the emission spectra of the NTs despite the presence of dozens of monolayers in their walls. We highlight the performance of the tubes as efficient optical resonators, whose confined optical modes strongly affect the emission bands. Finally, a brief conclusion is presented, along with an outlook of the future studies of this novel member of the family of radiative NTs, which have unique potential for different nanophotonics applications.

Published in: "arXiv Material Science".

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