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

High-Performance All 2D-Layered Tin Disulfide: Graphene Photodetecting Transistors with Thickness-Controlled Interface Dynamics

Tin disulfide crystals with layered two-dimensional (2D) sheets are grown by chemical vapor deposition using a novel precursor approach and integrated into all 2D transistors with graphene (Gr) electrodes. The Gr:SnS₂:Gr transistors exhibit excellent photodetector response with high detectivity and photoresponsivity. We show that the response of the all 2D photodetectors depends upon charge trapping at the interface and the Schottky barrier modulation. The thickness-dependent SnS₂ measurements in devices reveal a transition from the interface-dominated response for thin crystals to bulklike response for the thicker SnS₂ crystals, showing the sensitivity of devices fabricated using layered materials on the number of layers. These results show that SnS₂ has photosensing performance when combined with Gr electrodes that is comparable to other 2D transition metal dichalcogenides of MoS₂ and WS₂

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