No abstract

Among the candidates to replace Li-ion batteries, Li-S cells are an attractive option as their energy density is about five times higher (~2,600 Wh kg). The success of Li-S cells depends in large part on the utilization of metallic Li as anode material. Metallic lithium, however, is prone to grow parasitic dendrites and is highly reactive to several electrolytes; moreover, Li-S cells with metallic Li are also susceptible to polysulfides dissolution. Here, we show that ~10-nm-thick two-dimensional (2D) MoS can act as a protective layer for Li-metal anodes, greatly improving the performances of Li-S batteries. In particular, we observe stable Li electrodeposition and the suppression of dendrite nucleation sites. The deposition and dissolution process of a symmetric MoS-coated Li-metal cell operates at a current density of 10 mA cm with low voltage hysteresis and a threefold improvement in cycle life compared with using bare Li-metal. In a Li-S full-cell configuration, using the MoS-coated Li as anode and a 3D carbon nanotube-sulfur cathode, we obtain a specific energy density of ~589 Wh kg and a Coulombic efficiency of ~98% for over 1,200 cycles at 0.5 C. Our approach could lead to the realization of high energy density and safe Li-metal-based batteries.

Two-dimensional MoS2 is a promising material for next-generation electronic and optoelectronic devices due to its unique electrical and optical properties including the band gap modulation with film thickness. Although MoS2 has shown excellent properties, wafer-scale production with layer control from single to few layers has yet to be demonstrated. The present study explored the large-scale and thickness-modulated growth of atomically thin MoS2 on Si/SiO2 substrates using a two-step sputtering-CVD method. Our process exhibited wafer-scale fabrication and successful thickness modulation of MoS2 layers from monolayer (0.72 nm) to multilayer (12.69 nm) with high uniformity. Electrical measurements on MoS2 field effect transistors (FETs) revealed a p-type semiconductor behavior with much higher field effect mobility and current on/off ratio as compared to previously reported CVD grown MoS2-FETs and amorphous silicon (a-Si) thin film transistors. Our results show that sputter-CVD is a viable method to synthesize large-area, high-quality, and layer-controlled MoS2 that can be adapted in conventional Si-based microfabrication technology and future flexible, high-temperature, and radiation hard electronics/optoelectronics.

Two-dimensional (2D) van der Waal (vdW) heterostructures composed of vertically-stacked multiple transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS 2 ) and tungsten disulfide (WS 2 ) are envisioned to present unprecedented materials properties unobtainable from any other material systems. Conventional fabrications of these hybrid materials have relied on the low-yield manual exfoliation and stacking of individual 2D TMD layers, which remain impractical for scaled-up applications. Attempts to chemically synthesize these materials have been recently pursued, which are presently limited to randomly and scarcely grown 2D layers with uncontrolled layer numbers on very small areas. Here, we report the chemical vapor deposition (CVD) growth of large-area (>2 cm 2 ) patterned 2D vdW heterostructures composed of few layer, vertically-stacked MoS 2 and WS 2 . Detailed structural characterizations by Raman spectroscopy and high-resolution/scanning transmission electron microscopy (HRTEM/STEM) directly evidence the structural integrity of two distinct 2D TMD layers with atomically sharp vdW heterointerfaces. Electrical transport measurements of these materials reveal diode-like behavior with clear current rectification, further confirming the formation of high-quality heterointerfaces. The intrinsic scalability and controllability of the CVD method presented in this study opens up a wide range of opportunities for emerging applications based on the unconventional functionalities of these uniquely structured materials.The quest for the fundamental physics and exciting new phenomenon inherent to 2D TMDs has opened new avenues in the field of 2D vdW heterostructures [1][2][3] . Motivated by the well-established heterojunction engineering of traditional semiconductor thin films, developing new hybrid materials by stacking up dissimilar 2D TMDs allows for the realization of unique and superior materials properties that cannot be obtained otherwise 1,2 . For example, theoretical 4-10 and experimental [11][12][13][14][15][16][17][18][19][20][21] studies have demonstrated intriguing band alignment and tunneling transports as well as fast charge transfer and strong interlayer coupling in vertically-stacked 2D heterostructures employing molybdenum (Mo) or tungsten (W)-based TMDs. An important attribute of these atomically assembled hybrid materials is the seamless stitching of dissimilar 2D TMDs via weak vdW forces benefiting from relaxed lattice mismatch constriction 1 . The anisotropic bonding nature of the layered TMDs also enables them to grow aligning their 2D layers in two distinct directions [22][23][24] , further emphasizing the importance of controlling their morphology for desired materials functionalities. Thus, establishing reliable methods that can stack up multiple 2D TMDs with well-defined components and orientations will greatly broaden their horizons in a wide range of applications such as flexible electronics and optoelectronics utilizing their extraordinary opto-electrical properties and extr...

Two-dimensional (2D) materials have been a great interest as high-performance transparent and flexible electronics due to their high crystallinity in atomic thickness and their potential for variety applications in electronics and optoelectronics. The present study explored the wafer scale production of MoS 2 nanosheets with layer thickness modulation from single to multi-layer by using two-step method of metal deposition and CVD process. The formation of high-quality and layer thickness-modulated MoS 2 film was confirmed by Raman spectroscopy, AFM, HRTEM, and photoluminescence analysis. The layer thickness was identified by employing a simple method of optical contrast value. The image contrast in green (G) channel shows the best fit as contrast increases with layer thickness, which can be utilized in identifying the layer thickness of MoS 2. The presence of critical thickness of Mo for complete sulphurization, which is due to the diffusion limit of MoS 2 transformation, changes the linearity of structural, electrical, and optical properties of MoS 2. High optical transparency of >90%, electrical mobility of $12.24 cm 2 V À1 s À1 , and I on/off of $10 6 characterized within the critical thickness make the MoS 2 film suitable for transparent and flexible electronics as compared to conventional amorphous silicon (a-Si) or organic films. The layer thickness modulated large scale MoS 2 growth method in conjunction with the layer thickness identification by the nondestructive optical contrast will definitely trigger development of scalable 2D MoS 2 films for transparent and flexible electronics.