Atmospheric-pressure chemical vapor deposition (CVD) is used to grow monolayer MoS 2 two-dimensional crystals at elevated temperatures on silicon substrates with a 300 nm oxide layer. Our CVD reaction is hydrogen free, with the sulfur precursor placed in a furnace separate from the MoO 3 precursor to individually control their heating profiles and provide greater flexibility in the growth recipe. We intentionally establish a sharp gradient of MoO 3 precursor concentration on the growth substrate to explore its sensitivity to the resultant MoS 2 domain growth within a relatively uniform temperature range. We find that the shape of MoS 2 domains is highly dependent upon the spatial location on the silicon substrate, with variation from triangular to hexagonal geometries. The shape change of domains is attributed to local changes in the Mo:S ratio of precursors (1:>2, 1:2, and 1:<2) and its influence on the kinetic growth dynamics of edges. These results improve our understanding of the factors that influence the growth of MoS 2 domains and their shape evolution.

In recent years, impressive advances in harvesting renewable energy have led to a pressing demand for the complimentary energy storage technology. Here, a high Coulombic efficiency (∼99.7%) Al battery is developed using earth-abundant aluminum as the anode, graphite as the cathode, and a cheap ionic liquid analog electrolyte made from a mixture of AlCl 3 and urea in a 1.3:1 molar ratio. The battery displays discharge voltage plateaus around 1.9 and 1.5 V (average discharge = 1.73 V) and yielded a specific cathode capacity of ∼73 mAh g −1 at a current density of 100 mA g −1 (∼1.4 C). High Coulombic efficiency over a range of charge-discharge rates and stability over ∼150-200 cycles was easily demonstrated. In situ Raman spectroscopy clearly showed chloroaluminate anion intercalation/deintercalation of graphite (positive electrode) during charge-discharge and suggested the formation of a stage 2 graphite intercalation compound when fully charged. + cations. This battery is a promising prospect for a future high-performance, low-cost energy storage device.aluminum-ion battery | urea electrolyte | ionicity | ionic liquid | energy storage C heap, high-rate (fast charge/discharge) rechargeable batteries with long cycle lives are urgently needed for grid-scale storage of renewable energy, as it is becoming increasingly important to replace fossil fuels (1). Lithium-ion batteries (LIBs) are expensive and have limited cycle life, which makes them nonideal for grid-scale energy storage. Furthermore, high-rate capability is necessary for use in the grid, under which conditions LIBs become increasingly unsafe due to the flammability of the electrolytes used. Batteries based on aluminum offer a viable alternative due to aluminum's three-electron redox properties (offers potential for high-capacity batteries), stability in the metallic state, and very high natural abundance. Furthermore, the development of these batteries based on nonflammable electrolytes of low toxicity is critical for minimizing safety hazard and environmental impact. Recently, our group developed a secondary Al battery system based on the reversible deposition/stripping of aluminum at the Al negative electrode and reversible intercalation/deintercalation of chloroaluminate anions at the graphite positive electrode in a nonflammable 1-ethyl-3-methylimidazolium chloroaluminate (EMIC-AlCl 3 ) IL electrolyte (7,8). A ratio of AlCl 3 /EMIC = 1.3 by mole was used such that Al 2 Cl 7 − was present in the (acidic) electrolyte to facilitate aluminum deposition (9). During charging, Al 2 Cl 7 − is reduced to deposit aluminum metal, and AlCl 4 − ions intercalate (to maintain neutrality) in graphite as carbon is oxidized. During discharge, this battery exhibited a cathode specific capacity of ∼70 mAh g −1 with a Coulombic efficiency (CE) of 97-98%, and ultrahigh charge/discharge rate (up to 5,000 mA g −1 ) for over 7,000 cycles. However, room for improvement exists as the parameter space for the Al battery remains largely unexplored. The three-electron redox properti...

We show that controlling the introduction time and the amount of sulphur (S) vapour relative to the WO3 precursor during the chemical vapour deposition (CVD) growth of WS2 is critical to achieving large crystal domains on the surface of silicon wafers with a 300 nm oxide layer. We use a two furnace system that enables the S precursor to be separately heated from the WO3 precursor and growth substrate. Accurate control of the S introduction time enabled the formation of triangular WS2 domains with edges up to 370 μm which are visible to the naked eye. The WS2 domains exhibit room-temperature photoluminescence with a peak value around ∼635 nm and a full-width at half-maximum (FWHM) of ∼12 nm. Selected area electron diffraction from different regions of the triangular WS2 domains showed that they are single crystal structures.

Synthetic 2D crystal films grown by chemical vapor deposition are typically polycrystalline, and determining grain size within domains and continuous films is crucial for determining their structure. Here we show that grain boundaries in the 2D transition metal dichalcogenide WS2, grown by CVD, can be preferentially oxidized by controlled heating in air. Under our developed conditions, preferential degradation at the grain boundaries causes an increase in their physical size due to oxidation. This increase in size enables their clear and rapid identification using a standard optical microscope. We demonstrate that similar treatments in an Ar environment do no show this effect, confirming that oxidation is the main role in the structural change. Statistical analysis of grain boundary (GB) angles shows dominant mirror formation. Electrical biasing across the GB is shown to lead to changes at the GB and their observation under an optical microscope. Our approach enables high-throughput screening of as-synthesized WS2 domains and continuous films to determine their crystallinity and should enable improvements in future CVD growth of these materials.

Low-cost aluminum-graphite batteries utilizing an AlCl 3 /urea ionic liquid analog are promising candidates for grid-scale electricity storage. This work describes the use of N-methyl and N-ethyl derivatives of urea, which yield lower viscosity electrolytes, as a result of a lower ionic concentration. These electrolytes have increased ionic conductivities, and higher discharge voltages due to the differences in concentrations of electroactive species.

Understanding the stability of monolayer transition metal dichalcogenides in atmospheric conditions has important consequences for their handling, life-span, and utilization in applications. We show that cryogenic photoluminescence spectroscopy (PL) is a highly sensitive technique to the detection of oxidation induced degradation of monolayer tungsten disulfide (WS2) caused by exposure to ambient conditions. Although long-term exposure to atmospheric conditions causes massive degradation from oxidation that is optically visible, short-term exposure produces no obvious changes to the PL or Raman spectra measured at either room temperature or even cryogenic environment. Laser processing was employed to remove the surface adsorbents, which enables the defect states to be detected via cryogenic PL spectroscopy. Thermal cycling to room temperature and back down to 77 K shows the process is reversible. We also monitor the degradation process of WS2 using this method, which shows that the defect related peak can be observed after one month aging in ambient conditions.

Monolayer transition metal dichalcogenides (TMDs) are direct band gap semiconductors, and their 2D structure results in large binding energies for excitons, trions, and biexcitons. The ability to explore many-body effects in these monolayered structures has made them appealing for future optoelectronic and photonic applications. The band structure changes for bilayer TMDs with increased contributions from indirect transitions, and this has limited similar in-depth studies of biexcitons. Here, we study biexciton emission in bilayer WS2 grown by chemical vapor deposition as a function of temperature. A biexciton binding energy of 36 ±4 meV is measured in the as-grown bilayer WS2 containing 0.4% biaxial strain as determined by Raman spectroscopy. The biexciton emission was difficult to detect when the WS2 was transferred to another substrate to release the stain. Density functional theory calculations show that 0.4% of tensile strain lowers the direct band gap by about 55 meV without significant change to the indirect band gap, which can cause an increase in the quantum yield of direct exciton transitions and the emission from biexcitons formed by two direct gap excitons. We find that the biexciton emission decreases dramatically with increased temperature due to the thermal dissociation, with an activation energy of 26 ± 5 meV. These results show how strain can be used to tune the many-body effects in bilayered TMD materials and extend the photonic applications beyond pure monolayer systems.

We study the interactions in graphene/WS2 two-dimensional (2D) layered vertical heterostructures with variations in the areal coverage of graphene by the WS2. All 2D materials were grown by chemical vapor deposition and transferred layer by layer. Photoluminescence (PL) spectroscopy of WS2 on graphene showed PL quenching along with an increase in the ratio of exciton/trion emission, relative to WS2 on SiO2 surface, indicating a reduction in the n-type doping levels of WS2 as well as reduced radiative recombination quantum yield. Electrical measurements of a total of 220 graphene field effect transistors with different WS2 coverage showed double-Dirac points in the field effect measurements, where one is shifted closer toward the 0 V gate neutrality position due to the WS2 coverage. Photoirradiation of the WS2 on graphene region caused further Dirac point shifts, indicative of a reduction in the p-type doping levels of graphene, revealing that the photogenerated excitons in WS2 are split across the heterostructure by electron transfer from WS2 to graphene. Kelvin probe microscopy showed that regions of graphene covered with WS2 had a smaller work function and supports the model of electron transfer from WS2 to graphene. Our results demonstrate the formation of junctions within a graphene transistor through the spatial tuning of the work function of graphene using these 2D vertical heterostructures.