Controlled Preferential Oxidation of Grain Boundaries in Monolayer Tungsten Disulfide for Direct Optical Imaging

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

Electroluminescence Dynamics across Grain Boundary Regions of Monolayer Tungsten Disulfide

We study how grain boundaries (GB) in chemical vapor deposition (CVD) grown monolayer WS₂ influence the electroluminescence (EL) behavior in lateral source-drain devices under bias. Real time imaging of the WS₂ EL shows arcing between the electrodes when probing across a GB, which then localizes at the GB region as it erodes under high bias conditions. In contrast, single crystal WS₂ domains showed no signs of arcing or localized EL. Analysis of the eroded GB region shows the formation of micro- and nanoribbons across the monolayer WS₂ domains. Comparison of the EL spectrum with the photoluminescence spectrum from the monolayer WS₂ shows close agreement, indicating the EL emission comes from direct band gap excitonic recombination. These results provide important insights into EL devices that utilize CVD grown monolayer transition metal dichalcogenides when GBs are present in the active device region

Shape Evolution of Monolayer MoS₂ Crystals Grown by Chemical Vapor Deposition

Atmospheric-pressure chemical vapor deposition (CVD) is used to grow monolayer MoS₂ 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 MoO₃ precursor to individually control their heating profiles and provide greater flexibility in the growth recipe. We intentionally establish a sharp gradient of MoO₃ precursor concentration on the growth substrate to explore its sensitivity to the resultant MoS₂ domain growth within a relatively uniform temperature range. We find that the shape of MoS₂ 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 MoS₂ domains and their shape evolution

Revealing Defect-State Photoluminescence in Monolayer WS₂ by Cryogenic Laser Processing

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

Biexciton Formation in Bilayer Tungsten Disulfide

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

Doping Graphene Transistors Using Vertical Stacked Monolayer WS₂ Heterostructures Grown by Chemical Vapor Deposition

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