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.

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.

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.

Tailoring the Edge Structure of Molybdenum Disulfide toward Electrocatalytic Reduction of Carbon Dioxide.

Electrocatalytic conversion of carbon dioxide (CO2) into energy-rich fuels is considered to be the most efficient approach to achieve a carbon neutral cycle. Transition-metal dichalcogenides (TMDCs) have recently shown a very promising catalytic performance for CO2 reduction reaction in an ionic liquid electrolyte. Here, we report that the catalytic performance of molybdenum disulfide (MoS2), a member of TMDCs, can be significantly improved by using an appropriate dopant. Our electrochemical results indicate that 5% niobium (Nb)-doped vertically aligned MoS2 in ionic liquid exhibits 1 order of magnitude higher CO formation turnover frequency (TOF) than pristine MoS2 at an overpotential range of 50-150 mV. The TOF of this catalyst is also 2 orders of magnitude higher than that of Ag nanoparticles over the entire range of studied overpotentials (100-650 mV). Moreover, the in situ differential electrochemical mass spectrometry experiment shows the onset overpotential of 31 mV for this catalyst, which is the lowest onset potential for CO2 reduction reaction reported so far. Our density functional theory calculations reveal that low concentrations of Nb near the Mo edge atoms can enhance the TOF of CO formation by modifying the binding energies of intermediates to MoS2 edge atoms.

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.

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.

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.

Robust carbon dioxide reduction on molybdenum disulphide edges.

Electrochemical reduction of carbon dioxide has been recognized as an efficient way to convert carbon dioxide to energy-rich products. Noble metals (for example, gold and silver) have been demonstrated to reduce carbon dioxide at moderate rates and low overpotentials. Nevertheless, the development of inexpensive systems with an efficient carbon dioxide reduction capability remains a challenge. Here we identify molybdenum disulphide as a promising cost-effective substitute for noble metal catalysts. We uncover that molybdenum disulphide shows superior carbon dioxide reduction performance compared with the noble metals with a high current density and low overpotential (54 mV) in an ionic liquid. Scanning transmission electron microscopy analysis and first principle modelling reveal that the molybdenum-terminated edges of molybdenum disulphide are mainly responsible for its catalytic performance due to their metallic character and a high d-electron density. This is further experimentally supported by the carbon dioxide reduction performance of vertically aligned molybdenum disulphide.