A lithium-oxygen battery with a long cycle life in an air-like atmosphere.

Lithium-air batteries are considered to be a potential alternative to lithium-ion batteries for transportation applications, owing to their high theoretical specific energy. So far, however, such systems have been largely restricted to pure oxygen environments (lithium-oxygen batteries) and have a limited cycle life owing to side reactions involving the cathode, anode and electrolyte. In the presence of nitrogen, carbon dioxide and water vapour, these side reactions can become even more complex. Moreover, because of the need to store oxygen, the volumetric energy densities of lithium-oxygen systems may be too small for practical applications. Here we report a system comprising a lithium carbonate-based protected anode, a molybdenum disulfide cathode and an ionic liquid/dimethyl sulfoxide electrolyte that operates as a lithium-air battery in a simulated air atmosphere with a long cycle life of up to 700 cycles. We perform computational studies to provide insight into the operation of the system in this environment. This demonstration of a lithium-oxygen battery with a long cycle life in an air-like atmosphere is an important step towards the development of this field beyond lithium-ion technology, with a possibility to obtain much higher specific energy densities than for conventional lithium-ion batteries.

Mapping Thermal Expansion Coefficients in Freestanding 2D Materials at the Nanometer Scale.

Two-dimensional materials, including graphene, transition metal dichalcogenides and their heterostructures, exhibit great potential for a variety of applications, such as transistors, spintronics, and photovoltaics. While the miniaturization offers remarkable improvements in electrical performance, heat dissipation and thermal mismatch can be a problem in designing electronic devices based on two-dimensional materials. Quantifying the thermal expansion coefficient of 2D materials requires temperature measurements at nanometer scale. Here, we introduce a novel nanometer-scale thermometry approach to measure temperature and quantify the thermal expansion coefficients in 2D materials based on scanning transmission electron microscopy combined with electron energy-loss spectroscopy to determine the energy shift of the plasmon resonance peak of 2D materials as a function of sample temperature. By combining these measurements with first-principles modeling, the thermal expansion coefficients (TECs) of single-layer and freestanding graphene and bulk, as well as monolayer MoS_{2}, MoSe_{2}, WS_{2}, or WSe_{2}, are directly determined and mapped.

Direct Growth of High Mobility and Low-Noise Lateral MoS2 -Graphene Heterostructure Electronics.

Reliable fabrication of lateral interfaces between conducting and semiconducting 2D materials is considered a major technological advancement for the next generation of highly packed all-2D electronic circuitry. This study employs seed-free consecutive chemical vapor deposition processes to synthesize high-quality lateral MoS2 -graphene heterostructures and comprehensively investigated their electronic properties through a combination of various experimental techniques and theoretical modeling. These results show that the MoS2 -graphene devices exhibit an order of magnitude higher mobility and lower noise metrics compared to conventional MoS2 -metal devices as a result of energy band rearrangement and smaller Schottky barrier height at the contacts. These findings suggest that MoS2 -graphene in-plane heterostructures are promising materials for the scale-up of all-2D circuitry with superlative electrical performance.

Selective Ionic Transport Pathways in Phosphorene.

Despite many theoretical predictions indicating exceptionally low energy barriers of ionic transport in phosphorene, the ionic transport pathways in this two-dimensional (2D) material has not been experimentally demonstrated. Here, using in situ aberration-corrected transmission electron microscopy (TEM) and density functional theory, we studied sodium ion transport in phosphorene. Our high-resolution TEM imaging complemented by electron energy loss spectroscopy demonstrates a precise description of anisotropic sodium ions migration along the [100] direction in phosphorene. This work also provides new insight into the effect of surface and the edge sites on the transport properties of phosphorene. According to our observation, the sodium ion transport is preferred in zigzag edge rather than the armchair edge. The use of this highly selective ionic transport property may endow phosphorene with new functionalities for novel chemical device applications.

Bimodal Phonon Scattering in Graphene Grain Boundaries.

Graphene has served as the model 2D system for over a decade, and the effects of grain boundaries (GBs) on its electrical and mechanical properties are very well investigated. However, no direct measurement of the correlation between thermal transport and graphene GBs has been reported. Here, we report a simultaneous comparison of thermal transport in supported single crystalline graphene to thermal transport across an individual graphene GB. Our experiments show that thermal conductance (per unit area) through an isolated GB can be up to an order of magnitude lower than the theoretically anticipated values. Our measurements are supported by Boltzmann transport modeling which uncovers a new bimodal phonon scattering phenomenon initiated by the GB structure. In this novel scattering mechanism, boundary roughness scattering dominates the phonon transport in low-mismatch GBs, while for higher mismatch angles there is an additional resistance caused by the formation of a disordered region at the GB. Nonequilibrium molecular dynamics simulations verify that the amount of disorder in the GB region is the determining factor in impeding thermal transport across GBs.

Spatial Mapping of Hot-Spots at Lateral Heterogeneities in Monolayer Transition Metal Dichalcogenides.

Lateral heterogeneities in atomically thin 2D materials such as in-plane heterojunctions and grain boundaries (GBs) provide an extrinsic knob for manipulating the properties of nano- and optoelectronic devices and harvesting novel functionalities. However, these heterogeneities have the potential to adversely affect the performance and reliability of the 2D devices through the formation of nanoscopic hot-spots. In this report, scanning thermal microscopy (SThM) is utilized to map the spatial distribution of the temperature rise within monolayer transition metal dichalcogenide (TMD) devices upon dissipating a high electrical power through a lateral interface. The results directly demonstrate that lateral heterojunctions between MoS2 and WS2 do not largely impact the distribution of heat dissipation, while GBs of MoS2 appreciably localize heating in the device. High-resolution scanning transmission electron microscopy reveals that the atomic structure is nearly flawless around heterojunctions but can be quite defective near GBs. The results suggest that the interfacial atomic structure plays a crucial role in enabling uniform charge transport without inducing localized heating. Establishing such structure-property-processing correlation provides a better understanding of lateral heterogeneities in 2D TMD systems which is crucial in the design of future all-2D electronic circuitry with enhanced functionalities, lifetime, and performance.

Mapping Hot Spots at Heterogeneities of Few-Layer Ti3C2 MXene Sheets.

Structural defects and heterogeneities play an enormous role in the formation of localized hot spots in 2D materials used in a wide range of applications from electronics to energy systems. In this report, we employ scanning thermal microscopy (SThM) to spatially map the temperature rise across various defects and heterogeneities of titanium carbide (Ti3C2T x; T stands for surface terminations) MXene nanostructures under high electrical bias with sub-50 mK temperature resolution and sub-100 nm spatial resolution. We investigated several Ti3C2T x flakes having different thicknesses as well as heterogeneous MXene structures incorporating line defects or vertical heterojunctions. High-resolution temperature rise maps allow us to identify localized hot spots and to quantify the nonuniformity of the temperature fields across various morphological features. The results show that the local heating is most severe in vertical junctions of MXene flakes and is highly affected by nonuniform conduction due to the presence of line defects. These results provide a direct insight into the power dissipation of MXene-based devices and the roles of various heterogeneities that are inherent to the material synthesis process. This study provides a guideline for how a better understanding of the structure-property-processing correlations and further optimization of the synthesis routes could improve the lifetime, safety, and operation limits of the MXene-based devices.

New Class of Electrocatalysts Based on 2D Transition Metal Dichalcogenides in Ionic Liquid.

The optimization of traditional electrocatalysts has reached a point where progress is impeded by fundamental physical factors including inherent scaling relations among thermokinetic characteristics of different elementary reaction steps, non-Nernstian behavior, and electronic structure of the catalyst. This indicates that the currently utilized classes of electrocatalysts may not be adequate for future needs. This study reports on synthesis and characterization of a new class of materials based on 2D transition metal dichalcogenides including sulfides, selenides, and tellurides of group V and VI transition metals that exhibit excellent catalytic performance for both oxygen reduction and evolution reactions in an aprotic medium with Li salts. The reaction rates are much higher for these materials than previously reported catalysts for these reactions. The reasons for the high activity are found to be the metal edges with adiabatic electron transfer capability and a cocatalyst effect involving an ionic-liquid electrolyte. These new materials are expected to have high activity for other core electrocatalytic reactions and open the way for advances in energy storage and catalysis.

Stretching and Breaking of Ultrathin 2D Hybrid Organic-Inorganic Perovskites.

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) are recent members of the 2D materials family with wide tunability, highly dynamic structural features, and excellent physical properties. Ultrathin 2D HOIPs and their heterostructures with other 2D materials have been exploited for study of physical phenomena and device applications. The in-plane mechanical properties of 2D ultrathin HOIPs are critical for understanding the coupling between mechanical and other physical fields and for integrated devices applications. Here we report the in-plane mechanical properties of ultrathin freestanding 2D lead iodide perovskite membranes and their dependence on the membrane thickness. The in-plane Young's moduli of 2D HOIPs are smaller than that of conventional covalently bonded 2D materials. As the thickness increases from monolayer to three-layer, both the Young's modulus and breaking strength decrease, while three-layer and four-layer 2D HOIPs have almost identical in-plane mechanical properties. These thickness-dependent mechanical properties can be attributed to interlayer slippage during deformation. Our results show that ultrathin 2D HOIPs exhibit outstanding breaking strength/Young's modulus ratio compared to many other widely used engineering materials and polymeric flexible substrates, which renders them suitable for application into flexible electronic devices.

Power Dissipation of WSe2 Field-Effect Transistors Probed by Low-Frequency Raman Thermometry.

The ongoing shrinkage in the size of two-dimensional (2D) electronic circuitry results in high power densities during device operation, which could cause a significant temperature rise within 2D channels. One challenge in Raman thermometry of 2D materials is that the commonly used high-frequency modes do not precisely represent the temperature rise in some 2D materials because of peak broadening and intensity weakening at elevated temperatures. In this work, we show that a low-frequency E2g2 shear mode can be used to accurately extract temperature and measure thermal boundary conductance (TBC) in back-gated tungsten diselenide (WSe2) field-effect transistors, whereas the high-frequency peaks (E2g1 and A1g) fail to provide reliable thermal information. Our calculations indicate that the broadening of high-frequency Raman-active modes is primarily driven by anharmonic decay into pairs of longitudinal acoustic phonons, resulting in a weak coupling with out-of-plane flexural acoustic phonons that are responsible for the heat transfer to the substrate. We found that the TBC at the interface of WSe2 and Si/SiO2 substrate is ∼16 MW/m2 K, depends on the number of WSe2 layers, and peaks for 3-4 layer stacks. Furthermore, the TBC to the substrate is the highest from the layers closest to it, with each additional layer adding thermal resistance. We conclude that the location where heat dissipated in a multilayer stack is as important to device reliability as the total TBC.

Enhanced Thermal Boundary Conductance in Few-Layer Ti3 C2 MXene with Encapsulation.

Van der Waals interactions in 2D materials have enabled the realization of nanoelectronics with high-density vertical integration. Yet, poor energy transport through such 2D-2D and 2D-3D interfaces can limit a device's performance due to overheating. One long-standing question in the field is how different encapsulating layers (e.g., contact metals or gate oxides) contribute to the thermal transport at the interface of 2D materials with their 3D substrates. Here, a novel self-heating/self-sensing electrical thermometry platform is developed based on atomically thin, metallic Ti3 C2 MXene sheets, which enables experimental investigation of the thermal transport at a Ti3 C2 /SiO2 interface, with and without an aluminum oxide (AlOx ) encapsulating layer. It is found that at room temperature, the thermal boundary conductance (TBC) increases from 10.8 to 19.5 MW m-2 K-1 upon AlOx encapsulation. Boltzmann transport modeling reveals that the TBC can be understood as a series combination of an external resistance between the MXene and the substrate, due to the coupling of low-frequency flexural acoustic (ZA) phonons to substrate modes, and an internal resistance between ZA and in-plane phonon modes. It is revealed that internal resistance is a bottle-neck to heat removal and that encapsulation speeds up the heat transfer into low-frequency ZA modes and reduces their depopulation, thus increasing the effective TBC.

Anisotropic Friction of Wrinkled Graphene Grown by Chemical Vapor Deposition.

Wrinkle structures are commonly seen on graphene grown by the chemical vapor deposition (CVD) method due to the different thermal expansion coefficient between graphene and its substrate. Despite the intensive investigations focusing on the electrical properties, the nanotribological properties of wrinkles and the influence of wrinkle structures on the wrinkle-free graphene remain less understood. Here, we report the observation of anisotropic nanoscale frictional characteristics depending on the orientation of wrinkles in CVD-grown graphene. Using friction force microscopy, we found that the coefficient of friction perpendicular to the wrinkle direction was ∼194% compare to that of the parallel direction. Our systematic investigation shows that the ripples and "puckering" mechanism, which dominates the friction of exfoliated graphene, plays even a more significant role in the friction of wrinkled graphene grown by CVD. The anisotropic friction of wrinkled graphene suggests a new way to tune the graphene friction property by nano/microstructure engineering such as introducing wrinkles.

Characteristic Work Function Variations of Graphene Line Defects.

Line defects, including grain boundaries and wrinkles, are commonly seen in graphene grown by chemical vapor deposition. These one-dimensional defects are believed to alter the electrical and mechanical properties of graphene. Unfortunately, it is very tedious to directly distinguish grain boundaries from wrinkles due to their similar morphologies. In this report, high-resolution Kelvin potential force microscopy (KPFM) is employed to measure the work function distribution of graphene line defects. The characteristic work function variations of grain boundaries, standing-collapsed wrinkles, and folded wrinkles could be clearly identified. Classical and quantum molecular dynamics simulations reveal that the unique work function distribution of each type of line defects is originated from the doping effect induced by the SiO2 substrate. Our results suggest that KPFM can be an easy-to-use and accurate method to detect graphene line defects, and also propose the possibility to tune the graphene work function by defect engineering.

Cathode Based on Molybdenum Disulfide Nanoflakes for Lithium-Oxygen Batteries.

Lithium-oxygen (Li-O2) batteries have been recognized as an emerging technology for energy storage systems owing to their high theoretical specific energy. One challenge is to find an electrolyte/cathode system that is efficient, stable, and cost-effective. We present such a system based on molybdenum disulfide (MoS2) nanoflakes combined with an ionic liquid (IL) that work together as an effective cocatalyst for discharge and charge in a Li-O2 battery. Cyclic voltammetry results show superior catalytic performance for this cocatalyst for both oxygen reduction and evolution reactions compared to Au and Pt catalysts. It also performs remarkably well in the Li-O2 battery system with 85% round-trip efficiency and reversibility up to 50 cycles. Density functional calculations provide a mechanistic understanding of the MoS2 nanoflakes/IL system. The cocatalyst reported in this work could open the way for exploiting the unique properties of ionic liquids in Li-air batteries in combination with nanostructured MoS2 as a cathode material.

Nanostructured transition metal dichalcogenide electrocatalysts for CO2 reduction in ionic liquid.

Conversion of carbon dioxide (CO2) into fuels is an attractive solution to many energy and environmental challenges. However, the chemical inertness of CO2 renders many electrochemical and photochemical conversion processes inefficient. We report a transition metal dichalcogenide nanoarchitecture for catalytic electrochemical CO2 conversion to carbon monoxide (CO) in an ionic liquid. We found that tungsten diselenide nanoflakes show a current density of 18.95 milliamperes per square centimeter, CO faradaic efficiency of 24%, and CO formation turnover frequency of 0.28 per second at a low overpotential of 54 millivolts. We also applied this catalyst in a light-harvesting artificial leaf platform that concurrently oxidized water in the absence of any external potential.

Stable and Selective Humidity Sensing Using Stacked Black Phosphorus Flakes.

Black phosphorus (BP) atomic layers are known to undergo chemical degradation in humid air. Yet in more robust configurations such as films, composites, and embedded structures, BP can potentially be utilized in a large number of practical applications. In this study, we explored the sensing characteristics of BP films and observed an ultrasensitive and selective response toward humid air with a trace-level detection capability and a very minor drift over time. Our experiments show that the drain current of the BP sensor increases by ∼4 orders of magnitude as the relative humidity (RH) varies from 10% to 85%, which ranks it among the highest ever reported values for humidity detection. The mechanistic studies indicate that the operation principle of the BP film sensors is based on the modulation in the leakage ionic current caused by autoionization of water molecules and ionic solvation of the phosphorus oxoacids produced on moist BP surfaces. Our stability tests reveal that the response of the BP film sensors remains nearly unchanged after prolonged exposures (up to 3 months) to ambient conditions. This study opens up the route for utilizing BP stacked films in many potential applications such as energy generation/storage systems, electrocatalysis, and chemical/biosensing.

Chemical sensing with switchable transport channels in graphene grain boundaries.

Grain boundaries can markedly affect the electronic, thermal, mechanical and optical properties of a polycrystalline graphene. While in many applications the presence of grain boundaries in graphene is undesired, here we show that they have an ideal structure for the detection of chemical analytes. We observe that an isolated graphene grain boundary has ~300 times higher sensitivity to the adsorbed gas molecules than a single-crystalline graphene grain. Our electronic structure and transport modelling reveal that the ultra-sensitivity in grain boundaries is caused by a synergetic combination of gas molecules accumulation at the grain boundary, together with the existence of a sharp onset energy in the transmission spectrum of its conduction channels. The discovered sensing platform opens up new pathways for the design of nanometre-scale highly sensitive chemical detectors.