Ultrathin 2D Photodetectors Utilizing Chemical Vapor Deposition Grown WS₂ With Graphene Electrodes

In this report, graphene (Gr) is used as a 2D electrode and monolayer WS₂ as the active semiconductor in ultrathin photodetector devices. All of the 2D materials are grown by chemical vapor deposition (CVD) and thus pose as a viable route to scalability. The monolayer thickness of both electrode and semiconductor gives these photodetectors ∼2 nm thickness. We show that graphene is different to conventional metal (Au) electrodes due to the finite density of states from the Dirac cones of the valence and conduction bands, which enables the photoresponsivity to be modulated by electrostatic gating and light input control. We demonstrate lateral Gr–WS₂–Gr photodetectors with photoresponsivities reaching 3.5 A/W under illumination power densities of 2.5 × 10⁷ mW/cm². The performance of monolayer WS₂ is compared to bilayer WS₂ in photodetectors and we show that increased photoresponsivity is achieved in the thicker bilayer WS₂ crystals due to increased optical absorption. This approach of incorporating graphene electrodes in lateral TMD based devices provides insights on the contact engineering in 2D optoelectronics, which is crucial for the development of high performing ultrathin photodetector arrays for versatile applications

Utilizing Interlayer Excitons in Bilayer WS₂ for Increased Photovoltaic Response in Ultrathin Graphene Vertical Cross-Bar Photodetecting Tunneling Transistors

Here we study the layer-dependent photoconductivity in Gr/WS₂/Gr vertical stacked tunneling (VST) cross-bar devices made using two-dimensional (2D) materials all grown by chemical vapor deposition. The larger number of devices (>100) enables a statistically robust analysis on the comparative differences in the photovoltaic response of monolayer and bilayer WS₂, which cannot be achieved in small batch devices made using mechanically exfoliated materials. We show a dramatic increase in photovoltaic response for Gr/WS₂(2L)/Gr compared to monolayers because of the long inter- and intralayer exciton lifetimes and the small exciton binding energy (both interlayer and intralayer excitons) of bilayer WS₂ compared with that of monolayer WS₂. Different doping levels and dielectric environments of top and bottom graphene electrodes result in a potential difference across a ∼1 nm vertical device, which gives rise to large electric fields perpendicular to the WS₂ layers that cause band structure modification. Our results show how precise control over layer number in all 2D VST devices dictates the photophysics and performance for photosensing applications

Nanoporous Silicon-Assisted Patterning of Monolayer MoS₂ with Thermally Controlled Porosity: A Scalable Method for Diverse Applications

Nanoscale pore formation on chemical vapor deposition grown monolayer MoS₂ is achieved using oxygen plasma etching through a nanoporous silicon mask, creating round pores of ∼70 nm in diameter. The microscale areas with high porosity were successfully patterned via the usage of silicon masks. Thermal annealing in air after the pore formation in the monolayers results in the gradual enlargement of the pores, providing an effective method of controlling edge-to-area ratio of MoS₂ crystals. The photoluminescence of the nanoporous MoS₂ exhibits rapid increase and blue-shift due to facile p-doping during the thermal annealing process compared to pristine MoS₂. This method of fabricating porous transition metal dichalcogenide layers with controlled edge densities presents opportunities in various applications that require atomically thin nanomaterials with controlled pore density and edge sites, such as filtration, electrocatalysis, and sensing

Atomic Structure and Dynamics of Defects in 2D MoS₂ Bilayers

We present a detailed atomic-level study of defects in bilayer MoS₂ using aberration-corrected transmission electron microscopy at an 80 kV accelerating voltage. Sulfur vacancies are found in both the top and bottom layers in 2H- and 3R-stacked MoS₂ bilayers. In 3R-stacked bilayers, sulfur vacancies can migrate between layers but more preferably reside in the (Mo–2S) column rather than the (2S) column, indicating more complex vacancy production and migration in the bilayer system. As the point vacancy number increases, aggregation into larger defect structures occurs, and this impacts the interlayer stacking. Competition between compression in one layer from the loss of S atoms and the van der Waals interlayer force causes much less structural deformations than those in the monolayer system. Sulfur vacancy lines neighboring in top and bottom layers introduce less strain compared to those staggered in the same layer. These results show how defect structures in multilayered two-dimensional materials differ from their monolayer form

Epitaxial Templating of Two-Dimensional Metal Chloride Nanocrystals on Monolayer Molybdenum Disulfide

We demonstrate the formation of ionic metal chloride (CuCl) two-dimensional (2D) nanocrystals epitaxially templated on the surface of monolayer molybdenum disulfide (MoS₂). These 2D CuCl nanocrystals are single atomic planes from a nonlayered bulk CuCl structure. They are stabilized as a 2D monolayer on the surface of the MoS₂ through interactions with the uniform periodic surface of the MoS₂. The heterostructure 2D system is studied at the atomic level using aberration-corrected transmission electron microscopy at 80 kV. Dynamics of discrete rotations of the CuCl nanocrystals are observed, maintaining two types of preferential alignments to the MoS₂ lattice, confirming that the strong interlayer interactions drive the stable CuCl structure. Strain maps are produced from displacement maps and used to track real-time variations of local atomic bonding and defect production. Density functional theory calculations interpret the formation of two types of energetically advantageous commensurate superlattices <i>via</i> strong chemical bonds at interfaces and predict their corresponding electronic structures. These results show how vertical heterostructured 2D nanoscale systems can be formed beyond the simple assembly of preformed layered materials and provide indications about how different 2D components and their interfacial coupling mode could influence the overall property of the heterostructures

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

Atomically Flat Zigzag Edges in Monolayer MoS₂ by Thermal Annealing

The edges of 2D materials show novel electronic, magnetic, and optical properties, especially when reduced to nanoribbon widths. Therefore, methods to create atomically flat edges in 2D materials are essential for future exploitation. Atomically flat edges in 2D materials are found after brittle fracture or when electrically biasing, but a simple scalable approach for creating atomically flat periodic edges in monolayer 2D transition metal dichalcogenides has yet to be realized. Here, we show how heating monolayer MoS₂ to 800 °C in vacuum produces atomically flat Mo terminated zigzag edges in nanoribbons. We study this at the atomic level using an ultrastable in situ heating holder in an aberration-corrected transmission electron microscope and discriminating Mo from S at the edge, revealing unique Mo terminations for all zigzag orientations that remain stable and atomically flat when cooling back to room temperature. Highly faceted MoS₂ nanoribbon constrictions are produced with Mo rich edge structures that have theoretically predicted spin separated transport channels, which are promising for spin logic applications

Atomically Flat Zigzag Edges in Monolayer MoS₂ by Thermal Annealing

The edges of 2D materials show novel electronic, magnetic, and optical properties, especially when reduced to nanoribbon widths. Therefore, methods to create atomically flat edges in 2D materials are essential for future exploitation. Atomically flat edges in 2D materials are found after brittle fracture or when electrically biasing, but a simple scalable approach for creating atomically flat periodic edges in monolayer 2D transition metal dichalcogenides has yet to be realized. Here, we show how heating monolayer MoS₂ to 800 °C in vacuum produces atomically flat Mo terminated zigzag edges in nanoribbons. We study this at the atomic level using an ultrastable in situ heating holder in an aberration-corrected transmission electron microscope and discriminating Mo from S at the edge, revealing unique Mo terminations for all zigzag orientations that remain stable and atomically flat when cooling back to room temperature. Highly faceted MoS₂ nanoribbon constrictions are produced with Mo rich edge structures that have theoretically predicted spin separated transport channels, which are promising for spin logic applications

Atomically Flat Zigzag Edges in Monolayer MoS₂ by Thermal Annealing

The edges of 2D materials show novel electronic, magnetic, and optical properties, especially when reduced to nanoribbon widths. Therefore, methods to create atomically flat edges in 2D materials are essential for future exploitation. Atomically flat edges in 2D materials are found after brittle fracture or when electrically biasing, but a simple scalable approach for creating atomically flat periodic edges in monolayer 2D transition metal dichalcogenides has yet to be realized. Here, we show how heating monolayer MoS₂ to 800 °C in vacuum produces atomically flat Mo terminated zigzag edges in nanoribbons. We study this at the atomic level using an ultrastable in situ heating holder in an aberration-corrected transmission electron microscope and discriminating Mo from S at the edge, revealing unique Mo terminations for all zigzag orientations that remain stable and atomically flat when cooling back to room temperature. Highly faceted MoS₂ nanoribbon constrictions are produced with Mo rich edge structures that have theoretically predicted spin separated transport channels, which are promising for spin logic applications

Atomically Flat Zigzag Edges in Monolayer MoS₂ by Thermal Annealing

The edges of 2D materials show novel electronic, magnetic, and optical properties, especially when reduced to nanoribbon widths. Therefore, methods to create atomically flat edges in 2D materials are essential for future exploitation. Atomically flat edges in 2D materials are found after brittle fracture or when electrically biasing, but a simple scalable approach for creating atomically flat periodic edges in monolayer 2D transition metal dichalcogenides has yet to be realized. Here, we show how heating monolayer MoS₂ to 800 °C in vacuum produces atomically flat Mo terminated zigzag edges in nanoribbons. We study this at the atomic level using an ultrastable in situ heating holder in an aberration-corrected transmission electron microscope and discriminating Mo from S at the edge, revealing unique Mo terminations for all zigzag orientations that remain stable and atomically flat when cooling back to room temperature. Highly faceted MoS₂ nanoribbon constrictions are produced with Mo rich edge structures that have theoretically predicted spin separated transport channels, which are promising for spin logic applications